This is a term used to describe the difference between the Sidereal and Tropical Zodiac. There is a range of different Ayanamsa values commonly in use. The Sidereal Zodiac (fixed) excludes precession and keeps in step with the constellations (stars).
The difference between the starting point of the Tropical and Sidereal Zodiacs, due to the precession of the equinoxes. There are several in use in India today, but the official government-sanctioned Lahiri ayanamsa (22°27'37.7") is the most widely used. The other popular ayanamsas are Krishnamurti (22°21'50") and Raman (21°00'52") though it should be mentioned that there are at least 3 different versions of Lahiri, plus other lesser used models.
From the Hindu ayana, the arc that describes the increasing gap between the tropical and sidereal zodiacs. The ayanamsa, which changes continually at the rate of approximately 50" a year, is currently about 24°. To convert tropical measurements (based upon the Sun's Aries ingress) to sidereal measurements (based upon constellations, fixed stars) the appropriate ayanamsa is subtracted from the tropical position.
Difference between Sidereal and Tropical zodiacs
Ayanamsa is the Sanskrit term for the longitudinal difference between the tropical or Sayana and sidereal or Nirayana zodiacs. It is defined as the angle by which the sidereal ecliptic longitude of a celestial body is less than its tropical ecliptic longitude.