The Pali word nimitta literally means "sign." As was mentioned in the chapter in Right Concentration on Access Concentration, seeing a "diffused white light is a sign of good concentration." And as was mentioned in the chapter in Right Concentration on Entering the Jhānas, very subtle breathing or seemingly not even breathing are "signs that you've likely arrived at access concentration." The word nimitta does appear in the suttas and it has exactly this meaning in the context of meditation. It does get used in other contexts as well and also simply means "sign" in those places as well.1
The Vimuttimagga from approximately 100 CE says the following about the nimitta (sign) in the section Mindfulness of Respiration Procedure:
To the yogin who attends to the incoming breath with mind that is cleansed of the nine lesser defilements the nimitta arises with a pleasant feeling similar to that which is produced in the action of spinning cotton or silk cotton. Also, it is likened to the pleasant feeling produced by a breeze. Thus during breathing in and out, air touches the nose or the lip and causes the setting-up of air perception mindfulness. This does not depend on colour or form. This is called the nimitta. If the yogin develops the nimitta and increases it at the nose-tip, between the eyebrows, on the forehead or establishes it in several places, he feels as if his head were filled with air. Through increasing in this way his whole body is charged with bliss. This is called perfection.
And again, there is a yogin: he sees several nimittas from the beginning. He sees various forms such as smoke, mist, dust, sand of gold, or he experiences something similar to the pricking of a needle or to an ant's bite. If his mind does not become clear regarding these different nimittas, he will be confused. Thus he fulfils overturning and does not gain the perception of respiration. If his mind becomes clear, the yogin does not experience confusion. He attends to respiration and he does not cause the arising of other perceptions. ... If his mind is not disturbed, he will destroy the hindrances, and arouse the jhāna factors.2
(My underlining, my bolding.)
The phrase "he does not cause the arising of other perceptions" means you do not pay attention to these visual signs or to any tactile signs not associated with the touch of the breath. Hence you do not lose your attention to the breathing by generating "other perceptions," i.e., distractions. The underlined phrase specifically indicates mindfulness of breathing does not depend on anything visual; it's a tactile experience.
But as time went on, the word nimitta took on a new, more specific meaning. Four centuries later in the Visuddhimagga, under Mindfulness of Breathing, we find the following:
214. ...the nimitta soon appears to him. But it is not the same for all; on the contrary, some say that when it appears it does so to certain people producing a light touch like cotton or silk cotton or a draught.
215. But this is the exposition given in the commentaries: It appears to some like a star or a cluster of gems or a cluster of pearls, to others with a rough touch like that of silk-cotton seeds or a peg made of heartwood, to others like a long braid string or a wreath of flowers or a puff of smoke, to others like a stretched-out cobweb or a film of cloud or a lotus flower or a chariot wheel or the moon's disk or the sun's disk.3
(Again, my bolding.)
Compare the words in bold in the earlier Vimuttimagga description with those in bold in the Visuddhimagga. Notice how the quantity has increased and also notice the huge increase in the variety of visual nimittas. The Visuddhimagga goes on in verse 219 to say "Then he should fix his mind on that same sign; and so from now on, his development proceeds by way of fixing." In verse 221 and 222, we find "he should make it [the nimitta] grow and improve with repeated attention, and he should practice the ten-fold skill in absorption" and "as he strives thus, jhāna is achieved by him on that same sign." Now nimitta, and in particular the visual ones, have become the object of focused attention that brings one to full absorption and thence to the jhānas.
The following is from Nyanatiloka Mahathera's Buddhist Dictionary under the word nimitta:
'Mental (reflex-) image', obtained in meditation. In full clarity, it will appear in the mind by successful practice of certain concentration-exercises and will then appear as vividly as if seen by the eye. The object perceived at the very beginning of concentration is called the preparatory image (parikamma-nimitta). The still unsteady and unclear image, which arises when the mind has reached a weak degree of concentration, is called the acquired image (uggaha-nimitta). An entirely clear and immovable image arising at a higher degree of concentration is the counter-image (patibhāga-nimitta).
This transition from visual signs of good concentration that should be ignored to a visual sign becoming the heart of the method of absorption is a curious one. Bhikkhu Sona, in his essay "The Mystery of the Breath Nimitta,"4 suggests that a simile in the Paṭisambhidāmagga, a commentary from approximately 300 BCE, eventually was taken literally:
Whose mindfulness of breathing in and out is perfect, well developed, and gradually brought to growth according as the Buddha taught, It is he [who] illuminates the world just like the full moon free from cloud.5
Now certainly it is true that if you become VERY concentrated you might see an image that looks like "a star or a cluster of gems" or "chariot wheel or the moon's disk," etc. And it is also true that you can with sufficient concentration absorb into that experience so deeply that you no longer hear sounds, or are aware of your body, or are even aware of the passage of time. This is not what is described in the suttas; however, this is indeed what the later commentaries mean when they use the word "nimitta."
So we can say that in the suttas the word "nimitta" simply means "sign." And in the Visuddhimagga the word "nimitta" refers to a (usually visual) image that appears when your mind is sufficiently concentrated and which can be used as an object of full absorption. As you will see in the following appendix, in the Visuddhimagga a nimitta is required for jhāna practice without sufficient concentration to produce a nimitta, full absorption is not possible. Hence the jhānas, which for the Visuddhimagga are full absorption states, are not possible without a nimitta.
Of course the above discussion still leaves the question "Just what exactly causes a nimitta to appear?" Clearly it's a sign of deep concentration, but what is going on neurologically to cause a visual image to appear? A very interesting paper by Dr. Susan Blackmore,6 discussing the tunnel some people report upon returning from a near-death-experience, provides clues. She writes
In the 1930s, Heinrich Klüver, at the University of Chicago, noted four form constants in hallucinations: the tunnel, the spiral, the lattice or grating, and the cobweb. Their origin probably lies in the structure of the visual cortex, the part of the brain that processes visual information.
Brain activity is normally kept stable by some cells inhibiting others. Disinhibition (the reduction of this inhibitory activity) produces too much activity in the brain. This can occur near death (because of lack of oxygen) or with drugs like LSD, which interfere with inhibition.
It also seems that very deep states of meditation also interfere with inhibition. Dr. Blackmore and Tom Troscianko7 at the University of Bristol, UK, "used a computer to simulate what would happen when you have gradually increasing electrical noise in the visual cortex."
The most obvious thing about the representation in the cortex is that there are lots of cells representing the center of the visual field but very few for the edges. This means that you can see small things very clearly in the center, but if they are out at the edges you cannot. We took just this simple fact as a starting point.
The computer program starts with thinly spread dots of light, mapped in the same way as the cortex, with more toward the middle and very few at the edges. Gradually the number of dots increases, mimicking the increasing noise. Now the center begins to look like a white blob and the outer edges gradually get more and more dots. And so it expands until eventually the whole screen is filled with light. The appearance is just like a dark speckly tunnel with a white light at the end, and the light grows bigger and bigger (or nearer and nearer) until it fills the whole screen.
Nimittas do sometime appear as a tunnel8 although the star, cluster of gems, chariot wheel or the moon's disk seem to be more common. But these too would appear if the noise mentioned above does not increase above a certain threshold.
This means that steady visuals that you might see when meditating can be a good indication of deep concentration. The defused white light occurs in the early stages of disinhibition in the visual cortex. The concentration is providing the lack of inhibition and you can actually see that occurring. As disinhibition increases with further increases in concentration, the more the activity of the visual cortex comes to resemble a round object. This means you can actually track your depth of concentration by tracking what you see just don't get caught up in the visuals and lose your focus on your meditation object; if you do you'll start losing concentration. It's really important to "dance with the one that brung ya."
Just remember that no visuals of any sort are required to enter the jhānas described in the suttas; a visual nimitta is only necessary for the Visuddhimagga Jhānas which are discussed in the following appendix.
1. See Polak, 2011, pp 131-137 for a more detailed discussion of the use of the word "nimitta" in the suttas.
2. Vimuttimagga, 1995, p. 158f
3. Visuddhimagga, VIII, 214-215, 1991, p. 277f
4. http://www.arrowriver.ca/dhamma/nimitta.html and mirrored at http://leighb.com/case_of_the_missing_simile.htm
5. Paṭisambhidāmagga III.171, p. 172
6. Susan Blackmore, Near-Death Experiences: In or out of the body?, Skeptical Inquirer 1991, pp 16, 34-45.
7. Blackmore, S. J., and T. S. Troscianko. 1989. The physiology of the tunnel. Journal of Near-Death Studies, 8:15-28.
8. Mentioned in meditation instructions given by Ven. Pa Auk, July-Oct, 2011, IMS Forest Refuge, Barre, MA