1. Most important; principal. See synonyms at chief.
2. Exerted to the utmost; sheer: by main strength.
3. Nautical. Connected to or located near the mainmast: a main skysail.
4. Grammar. Of, relating to, or being the principal clause or verb of a complex sentence.
5. Obsolete. Of or relating to a continuous area or stretch, as of land or water.

1. The chief or largest part: His ideas are, in the main, impractical.
2. The principal pipe or conduit in a system for conveying water, gas, oil, or other utility.
3. Physical strength: fought with might and main.
4. A mainland.
5. The open ocean.
6. Nautical.
   1. A mainsail.
   2. A mainmast.

[Middle English, from Old English mægen, strength.]

The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.