1) A players health is such that upon being killed in a multiplayer game they explode into many gibs and giblets (peices of simulated flesh and gore). (Quake 1)
2) A game mode where every weapon (usually only a hitscan railgun/ASMD) instantly kills the player in one shot. (Started in Quake 2 and popularised as a mutator in Unreal Tournament)
3) Finding yourself at the bottom of a chasm/lava/force field and being instantly turned into gibs when you touch the bottom.
I was playng instagib the other day when I totally owned some guys when they lined up and I instagib'ed 4 people in a row!
So I was running around and a spammed a rocket into a door and it slapped this poor sucker in the face. I then jumped off the top of the level and instagibbed him with my railgun as I was falling. Dont think he had time to blink!
Spawn rape is specifically defined as the act of a person of having their crosshair or damage radius of a projectile aimed specifically at the place where a persons model or character spawn. This act of camping a particular spawn and then instantly killing the opponent or putting them at a large damage defecit from their initial spawn health as they respawn is spawn rape.
Spawn rape has been in multiplayer games from their very inception.
It is the map designers job to prevent spawn raping by designing levels that allow the player to move significantly from a protected spawn in order to be able to retaliate and be able to fire back on a camper, thus preventing spawn rape.
OMGZ! That guy is just camping right in front of that corner spawn and killing them instantly. SPAWN RAPE!!!