One reasonable criterion seems to be the ratio of number of games with alternative move N and alternative move N+1, where alternative move N+1 is the alternative move played more rarely than alternative move N, but more often than any other alternative move which is played less often than alternative move N. Following this criterion and my database, there seem to be three reasonable "cut-off-points" between "mainline" and "offbeat":

(i) 3...Nf6 (22,521 games), compared to 3...a6 (140,731 games), ratio appr. 1 : 6.5,

(ii) 3...Bb4 (363 games), compared to 3...g6 (2,936 games), ratio appr. 1 : 8, and

(iii) 3...Qg5 or 3...Nh6 (1 game each), compared to 3...Nb8 (7 games), ratio = 1 : 7.

So a reasonable cut-off-point both from a statistical point of view and a common-sense point of view seems to be number (ii): 3...g6 is the rarest "mainline" move, 3...Bb4 the most common "offbeat" move. And yes, 3...a5, 3...f6, and 3...d5 would be offbeat too.

Zwischenzugzwang