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.