1seal 
/ˈsi:l/ verb
seals; sealed; seal·ing /ˈsi:l/ verb 1 a [+ obj] : to close (something) tightly so that air, liquid, etc., cannot get in or out
▪ He sealed the jar. — often + up ▪ He sealed up the cracks in the wall.
b [+ obj] : to close (an envelope, bag, etc.) by sticking or pressing two of its parts together
▪ Would you seal this envelope?
c [no obj] : to become closed tightly
▪ Make sure the bag seals properly. 2 [+ obj] : to cover the surface of (something) with a substance that will protect it
▪ We used a sealant/sealer to seal the wood to make it waterproof. 3 [+ obj] : to make (something) definite and final
▪ His home run sealed the victory. [=his home run made it certain that his team would win] ▪ (informal) They finally sealed the deal. [=they finally reached an agreement and made a deal] ▪ Their decision sealed her fate. = Once they made their decision, her fate was sealed. [=their decision made it certain that something bad or unwanted would happen to her] 4 [+ obj] : to prevent someone from going into or through (an area or place)
▪ Troops have sealed the border between the two countries. ▪ The room/chamber has been sealed and no one can enter.
▪ He sealed the jar. — often + up ▪ He sealed up the cracks in the wall.
b [+ obj] : to close (an envelope, bag, etc.) by sticking or pressing two of its parts together
▪ Would you seal this envelope?
c [no obj] : to become closed tightly
▪ Make sure the bag seals properly.
▪ We used a sealant/sealer to seal the wood to make it waterproof.
▪ His home run sealed the victory. [=his home run made it certain that his team would win] ▪ (informal) They finally sealed the deal. [=they finally reached an agreement and made a deal] ▪ Their decision sealed her fate. = Once they made their decision, her fate was sealed. [=their decision made it certain that something bad or unwanted would happen to her]
▪ Troops have sealed the border between the two countries. ▪ The room/chamber has been sealed and no one can enter.
my lips are sealed — see lip
seal in [phrasal verb]
seal (something) in or seal in (something) : to prevent (something that is in something else) from getting out or escaping
▪ He seared the steak to seal in the juices.
▪ He seared the steak to seal in the juices.
seal off [phrasal verb]
seal off (something) or seal (something) off : to prevent people from entering or leaving (an area or place)
▪ Police sealed off the crime scene.
▪ Police sealed off the crime scene.
— sealed adjective
▪ a sealed jar
▪ a sealed jar
