(Ice) hockey: A change of players which occurs 'on the fly', that is, without a stoppage in play. This usually happens when the puck has been dumped by the offensive team into the opponents' zone, allowing that team, and also often the defenders, to bring on a fresh shift of players. There is some risk, in that the opponents could mount an offensive rush while the change is in progress and the players are out of position to defend.
Hockey players usually play a 60 to 90 second shift before changing. If they are trapped in the defensive zone for longer than that, as on a penalty kill, dumping the puck out to get a change becomes a top priority.
Goaltenders do not usually change on the fly, as they don't play shifts and have a long way to go for a change. Scottie Bowman was the first, and perhaps only, NHL coach to change goalies on the fly, while with the Pittsburgh Penguins. He did so towards the end of the final game of the season, meaningless in the standings. He'd promised the CBC broadcast team an unusual play, and he delivered. The Penguins rushed up the ice on offense, and the goalie lumbered after. He threw himself onto the boards at the Penguins' bench, and was hauled across by laughing players as the backup goalie went out the door and bolted for the goal. If memory serves, Ken Wregget was one of the 'tenders involved. (Yes, the change was successful. I wouldn't want to see it done with a game's results on the line, though.)