Vedic astrology glossary
What is Ayanamsa?
Ayanamsa is the angular difference between the tropical and sidereal zodiacs, caused by the precession of the equinoxes. Vedic astrology applies it to convert tropical positions to sidereal.
The Lahiri (Chitrapaksha) ayanamsa is the Indian government standard and RishiAstro's default; Raman and KP (Krishnamurti) are other common choices. The ayanamsa you pick shifts every sidereal position, so it can change signs and nakshatras near boundaries.