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 / side-by-side smooth bore. (a side-by-side "musket" (unrifled) longarm)

Source: wiktionary.org