What is the difference between a firearm and a sidearm?

Answer 1

A firearm is any weapon that expels a bullet by explosive decompression. These are typically known as guns, but not all guns are firearms. some guns, such as CO2 or pumped air powered rifles and pistols are not considered firearms.

a sidearm is typically a smaller secondary weapon used when your primary weapon either fails, breaks, runs out of ammo, or it can be used along with the primary weapon. Sidearms generally include shorter swords, daggers, knives, and pistols. Many swordfighters in medieval times would wield a 1 handed sword with a parrying dagger (also known as a Main Gauche (french for left handed), the dagger would be used to deflect incoming blows so that their sword is free to strike back.

in the military soliders often have a pistol and combat knife, among other weapons, to go along with their combat rifle. the pistol is often called the sidearm. If your rifle jams or runs out of ammunition, you can draw your pistol and shoot so that you're not completely defenseless. The pistol is issues because 2 rifles would be too unwieldly and heavy and swords make poor weapons against guns, which are the go to weapon for modern infantry.