close envelope1 seal something (up/down) to close an envelope, etc. by sticking the edges of the opening togetherMake sure you've signed the cheque before sealing the envelope.a sealed bid(= one that is kept in a sealed envelope and therefore remains secret until all other bids have been received)
close container2 [often passive] seal something (up) (with something) to close a container tightly or fill a crack, etc, especially so that air, liquid, etc. cannot get in or outThe organs are kept in sealed plastic bags.
cover surface3 [often passive] seal something (with something) to cover the surface of something with a substance in order to protect itThe floors had been stripped and sealed with varnish.
make something definite4 seal something to make something definite, so that it cannot be changed or argued aboutto seal a contractThey drank a glass of wine to seal their new friendship.The discovery of new evidence sealed his fate(= nothing could prevent what was going to happen to him).She sealed victory with a birdie at the final hole.
close borders/exits5 seal something (of the police, army, etc.) to prevent people from passing through a placeTroops have sealed the borders between the countries.
Idiomssee my lips are sealed at lip, signed and sealedsigned, sealed and delivered at sign v.