Survivor encounters are some special events who occur in Resident Evil 2, Resident Evil 3: Nemesis, Resident Evil 5 (in the civilian checkpoint chapter), and Resident Evil 6. They feature survivors who appear briefly in the game, under attack from a particular creature - usually a zombie. In all encounters of Nemesis, 5, and most of 6, the survivors cannot be saved and their deaths are unavoidable.
Also, in Resident Evil Outbreak, exist some survivor who you will encounter during the game. Some of them (as Regan Mallet or Patrick Reyes) can be helped in sidequests and this give prizes for the players, others (as Matthew) can be saved by the player, but this does not result in anything.