Yajña is the Vedic sacrifice, in which offerings are poured into the sacred fire with mantras to the gods. The ritual was believed to sustain the cosmic order. The Bhagavad Gītā widens the idea: all action done as an offering, without desire for its fruit, is sacrifice and does not bind.
Yajña
Sacrifice: the Vedic fire ritual, and later any act offered to God.
Also: Yajna, sacrifice