A tic is each time unit in-game. By default 1 second equals 30 tics.
From the EDuke32 source code:
// this used to be TICRATE/GAMETICSPERSEC, which was 120/26 = 4.615~ truncated to 4 by integer division #define TICSPERFRAME 4 #define TICRATE 120
Because the word "tic" can mean a variety of other timing units, the less ambigious "game tic" should be preferred.