Plain English Meaning
Instead of mixing fuel and air in one place and sending the mixture to all the cylinders, a fuel injection system sprays a measured amount of fuel directly at each cylinder, and the air arrives separately. Each cylinder gets its own little fuel nozzle.
Full Definition
An engine fuel delivery system that meters fuel and sprays it directly into the intake port of each cylinder, rather than mixing it with air in a carburetor upstream. Fuel is pressurized and distributed through a fuel pump, fuel/air control unit, fuel manifold (distributor), and individual discharge nozzles at each cylinder. Air enters separately through the induction system and combines with the fuel just before entering the combustion chamber.
Context Anchor
Seen in engine and induction system discussions, , engine- , and for starting or rough .