in the main Definition

main
adj

1. Most important; chief; leading.

Example: main event
Thesaurus: principal, chief, foremost, leading, cardinal, central, preeminent, primary, essential, vital, crucial; Antonym: minor.
2. literary
Extreme; utmost; sheer.
Example: main force

noun

1. The chief pipe, conduit or cable in a branching system.

Example: a burst water main
Example: not connected to the mains
Thesaurus: pipe, conduit, trough, channel, duct, line.
Form: the mains (often)
(chiefly Brit)
2. The network by which power, water, etc is distributed.

Example: The electricity board came to connect us up with the mains
Form: the mains (usually)
3. Belonging or relating to the mains (senses 1 and 2).

Example: mains supply
Form: mains (always)
4. old use
Great strength, now only usually in the phrase with might and main. See under might².
5. old use
The open sea.
Example: the Spanish main

Idiom: in the main

Mostly; on the whole.

Etymology: Anglo-Saxon mægen (as noun) in obsolete sense ‘strength'; partly from Norse meginn strong.