- (usually before the noun) The main thing is the most important or biggest one.
- They turned off the main road and saw a small house.
- There are many problems, but the main one is money.
- what was the main reason for war in Europe in 1914?
- (countable) A main is a large pipe or wire that carries gas, water, electricity, etc.
- A break in a water main has closed the road.