ZPA was a Game Mode in Zombie Panic: Source which was removed in version 3.0. ZPA maps had the zpa_ prefix.
About[]
ZPA mode places two even teams of survivors and zombies against each other to compete for control of areas and objectives. These objectives vary from map to map, as they may be goals such as the survivors defending a location, or the zombies preventing the humans from achieving something, similar to Objective Mode.
What makes this game mode unique is how there is no infecting (the percent chance for infecting a survivor has been replaced with a percent chance to bleed the player to death), zombification, or life count. This means the humans who are killed will still respawn as humans, and zombies who are killed will still respawn as zombies without any consequences.
The round is won if one team successfully thwarts the other teams' goal (for example, on zpa_town the human can win by capturing a point before the zombies do).
There were only two official maps for the ZPA gamemode, zpa_town and zpa_badbayou.
So far, nobody seems to know what the A in ZPA stands for. The community has jokingly suggested that it is an acronym for Zebra Panda Alliance, but it may be more likely that it stands for Arena.