Ayanamsa is the precession-driven gap between the tropical zodiac (used in Western astrology, fixed to seasons) and the sidereal zodiac (used in Vedic astrology, fixed to stars). The two were aligned around 285 CE; they now differ by approximately 24 degrees. Vedic computations subtract the ayanamsa from tropical longitudes to get sidereal positions. The most common standard is Lahiri (Chitrapaksha), adopted by the Indian government in 1955. Other ayanamsas — Raman, Krishnamurti, Fagan-Bradley — differ by less than a degree, but the choice matters for divisional charts and nakshatra placement.
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Ayanamsa
The angular gap between the tropical zodiac and the sidereal zodiac.