Attacking people from behind (or otherwise without them noticing you) either allows you to instantly kill them or deal significantly higher amount of damage.
Alternate name: Stealth attack
The first video game about Backstabbing was released on November 30, 1998.
This can fail realism either by causing critical hit or giving other bonus damage, which usually happens in statistics based games (many RPGs or those influenced by them), or by killing them instantly with a weapon hit but not doing the same on fully aware target (e.g. crossbow bolt to unprotected head of unaware opponent kills them outright, but if they're aware of you or even suspecting on your presence, it may require 2-3 bolts to kill them).