Real Dictionary

tap
countable noun: A tap is a device that controls the flow of a liquid or gas from a pipe or container, for example on a sink.
verb: If you tap something, you hit it with a quick light blow or a series of quick light blows.

tap in American English 1
to strike lightly and rapidly
intransitive verb: to strike lightly but audibly, as to attract attention
noun: a light but audible blow
transitive verb: to strike with a light but audible blow or blows; hit with repeated, slight blows

tap in American English 2
a device for starting or stopping the flow of liquid in a pipe, barrel, etc.; faucet
to put a tap or spigot on
noun: a cylindrical stick, long plug, or stopper for closing an opening through which liquid is drawn, as in a cask; spigot

tap in Mechanical Engineering
verb: If you tap an object or material, you cut a female screw thread in it using a specially designed tool.