What is the meaning of Side-by-side?

Adjacent; juxtaposed.

A type of multipurpose off-highway utility vehicle, with four wheels, resembling a club cart or a golf cart

Synonym of double-barreled shotgun. Ellipsis of side-by-side shotgun.

Synonym of double rifle. Ellipsis of side-by-side rifle.

Ellipsis of side-by-side smoothbore rifle, side-by-side smooth-bore rifle, side-by-side smoothbore, or side-by-side smooth bore (a side-by-side "musket" (unrifled) longarm).

Source: wiktionary.org