Noun
a firearm that is held and fired with one hand
Source: WordNetSome Mexican police officers working in urban environments carry both a 9 mm machine pistol and a standard semi-automatic side arm. Source: Internet
Master Sergeant Gordon used his long range rifle and side arm to kill an undetermined number of attackers until he depleted his ammunition. Source: Internet
Sergeant First Class Shughart used his long range rifle and side arm to kill an undetermined number of attackers while traveling the perimeter, protecting the downed crew. Source: Internet
Some kinds of swords are still commonly used today as weapons, often as a side arm for military infantry. Source: Internet