To put it simple, in the context of samadhi, nimitta is a perceptual image of the basis of concentration. When you keep your attention on that basis (say, air), your individual perceptual image of it kind of crystallises into something directly perceptible.
It can be tactile, or it can be visual, it does not matter.
Any perceptual image you keep in mind flavors your perception. For example, if you remember a gentle breeze, you can feel it a little right now. Similar things happen in meditation, but in intensified form.
In the case of Anapanasati, the basis of concentration is the air element, as in the case of air kasina practice.
Kiṃ pana pathavīkasiṇaṃ ādiṃ katvā aṭṭhikasaññāpariyosānāvesā rūpāvacarappanā, udāhu aññāpi atthīti? Atthi; ānāpānajjhānañhi kāyagatāsatibhāvanā ca idha na kathitā. Kiñcāpi na kathitā vāyokasiṇe pana gahite ānāpānajjhānaṃ gahitameva; vaṇṇakasiṇesu ca gahitesu kesādīsu catukkapañcakajjhānavasena uppannā kāyagatāsati, dasasu asubhesu gahitesu dvattiṃsākāre paṭikūlamanasikārajjhānavasena ceva navasivathikāvaṇṇajjhānavasena ca pavattā kāyagatāsati gahitāvāti. Sabbāpi rūpāvacarappanā idha kathitāva hotīti.
"But is this all the absorption belonging to the consciousness of the sphere of refined form, beginning with the earth kasiṇa and ending in the perception of the skeleton? Or is there anything else?"
"Yes, there is. There is ānāpāna jhāna and the development of kāyagatāsati, which have not been spoken of here."
"Because ānāpāna jhāna is included in the air kasiṇa; the development of kāyagatāsati arisen by virtue of the fourfold and fivefold jhānas with reference to the hair etc., is included in the colour kasiṇas; the kāyagatāsati produced by virtue of the jhānas attending to the unattractiveness in the thirty-two parts of the body, and that of the jhāna attending to the colours of the nine kinds of corpses in the charnel grounds is included in the ten repulsive things. Thus all the absorptions of consciousness connected with the sphere of refined form have been included here."
More about 'nimitta' in the thread:http://www.lioncity.net/buddhism/index. ... opic=17871