Repetition of any Beat, or rhythmic phrase (Bol or a group of Bol) three times and ends on Sum is Tihai.  Tihai can begin on any beat (Matra) of Taal but always ends on Sum.  It can be as small as repetition of a beat 1,1,1 or dha dha dha or it can be take entire Avartan.

simple example: Trital 8 Matra Laya

(tathaithaitat aathaithai thai) x 3