Illegal Move??

I've played chess for years but have never seen a pawn move as it did in the below game --- see move #34 (black). Is this a legal move? Or just a chess.com quirk?

Please enlighten...

I forget it has a french name.

En passant has a interesting history. Back in the old days, pawns would only be allowed 1 square ahead at all times. So having a white pawn on the 5 rank or a black pawn on the 4 rank was a good tactic, because whenever a starting pawn move out 1 square, I would get taken, loosening up the opponent's pawn structure. But, as we all know, pawns were eventually allowed 2 squares in the opening, so If there was a pawn on the 4 (for black) or 5 (for white) ranks (as I said earlier), the opponent could just push the pawn 2 squares, potentially avoiding the danger of that 4 or 5 pawn. But the players who liked to use that tactic of putting pawns on the 4 or 5 squares thought that it wasn't fair that their opponent could then cheaply avoid their 4 or 5 pawn by pushing one of their starting pawns 2 squares, rendering their 4 or 5 pawn useless.

So, Instead of abolishing the "2-square head start" rule for the starting pawn, they (whoever they is) simply invented en passant, the strange move we find in aslalaji's game at the top.

Gypsum4 explained it well.

I will just add this: We used to call it "the fifth-row move" growing up. When I explain it to new players, I find that this name makes it easier to remember.

When you have a pawn on the fifth row (from your side of the board), and the opponent tries to pass it with a 2-square pawn move, you have one chance to take it.

Chess is a game of war, and I like to think of en passant as one soldier trying to run past the enemy soldier. The enemy soldier just sticks his sword out and gets him as he runs by. :)

=)