A river in southern Germany, flowing from Bavaria to the Rhine.

A river in Northern Ireland, flowing into Lough Neagh.

Ellipsis of Main Street.: A street named "Main Street".

Ellipsis of Main Street (“main street”).: A central street of a community

The noun "main" refers to that which is chief or principal, the chief or main portion, the bulk or greater part, and can also represent the primary character one plays in a video game where multiple characters are available.

A large cable or pipe providing utility service to an area or a building, such as a water main or electric main.

(informal) Short for main course (“the principal dish of a meal”).

(now poetic) The high seas.

(nautical) Short for mainsail.

A banker's shovel for coins.

(transitive, slang) Short for mainline (“to inject (a drug) directly into a vein”).

(transitive, gaming) To mainly play a specific character or side, or with specific equipment, during a game.

Of chief or leading importance; prime, principal.

Chief, most important, or principal in extent, size, or strength; consisting of the largest part.

(nautical) Belonging to or connected with the principal mast in a vessel.