Definition of "Tube" :

noun: an electric railway operating below the surface of the ground (usually in a city)

"In Paris the subway system is called the metro' and in London it is called the tube' or the underground'."

noun: conduit consisting of a long hollow object (usually cylindrical) used to hold and conduct objects or liquids or gases

noun: electronic device consisting of a system of electrodes arranged in an evacuated glass or metal envelope

noun: (anatomy) any hollow cylindrical body structure

noun: a hollow cylindrical shape

verb: place or enclose in a tube

verb: ride or float on an inflated tube

"We tubed down the river on a hot summer day."

verb: convey in a tube

"Inside Paris, they used to tube mail."

verb: provide with a tube or insert a tube into