Learner's definition of TUBE
1 a : a long, hollow object that is used especially to control the flow of a liquid or gas b : an object shaped like a pipe
a tube of lipstick
a cardboard tube
2 [count] : a soft, long, narrow container that has a small opening at one end and that contains a soft material which can be pushed out by squeezing 3 [count] : a thin, long, hollow part within an animal or plant
the bronchial tubes
a pollen tube
4 the tube US, informal : the television 5 [noncount], British : the system of trains that run underground in London
It's easy to get around London on the tube.
There's a tube station a couple of blocks away from here.
She travels by tube to work every day.
go down the tubesinformal
1 : to fail or become ruined