Plain English Meaning
It is a small nozzle that squirts a measured amount of fuel into each cylinder so the engine gets the right fuel-to-air mix at every cylinder, every time.
Full Definition
A fuel injector is a device that delivers a metered spray of fuel directly into the intake port of each cylinder, or in some systems directly into the cylinder itself, instead of relying on a carburetor to mix fuel and air in a single central location.
Context Anchor
Seen in discussions of engine fuel systems, diesel-style aircraft engines, and two-stroke compression-ignition engine operation.
Derivation
From Latin 'injicere', meaning 'to throw in'. The injector literally throws (sprays) fuel into the intake stream — a useful image for what the device actually does.