For a gamemode on SAS: Zombie Assault 4, see Nightmare Mode (SAS4)
Nightmare Mode is a game mode in SAS: Zombie Assault 3 multiplayer. Being unlocked at Rank 40, this is the way of the game saying that it's time to go elite; Nightmare Mode is one of the only ways to get XP at this point (along with Purge on regular mode). Nightmare mode does not exist in the mobile version, and the player can rank up by gaining normal XP.
On Nightmare, the zombies seem to be stronger, faster, larger, and more resilient. There also appears to be more of them spawning, which means they can easily overpower a player who is alone, be it by their choice or the party leaving. Zombies, of course, have increased resistances, meaning that it will require sufficiently increased quantities of ammo to kill them.