One complete pass through the cycle, from beat 1 to the next beat 1.
One full statement of a tala, sam to sam. The avartan is the unit in which repertoire is measured: a kaida theme might occupy one avartan, a chakradhar three, a peshkar development many.
Thinking in whole avartans rather than individual beats is a marker of maturity. Accompanists track the soloist's phrase across the avartan and shape the theka toward the coming sam instead of counting matra by matra.
Further reading
Martin Clayton, Time in Indian Music: Rhythm, Metre, and Form in North Indian Rag Performance (Oxford University Press, 2000).
