The prefix at-, when positioned before consonants "d" ant "t", takes the form of ati-, e.g. atiduoti ('give back'), atidėti ('postpone'), atitikti ('match').

Jis atiduoda man automobilį. He gives me back the car.
Dėstytojas atideda paskaitą. The lecturer postones the lecture.

This is done for easier pronunciation, much like prefix ap- turning into api- before consonants "b" and "p", e.g. apibėgti, apiplaukti.