A "Stroker" engine is one which uses an aftermarket or modified crankshaft, connecting rods, and/or pistons to increase the stroke of the motor, which is the linear distance traveled by the pistons.
"Stroking" an engine increases the displacement. Most stroker engines use kits which contain components much stronger than stock, making the completed engine more durable as well as more powerful.
This modification is generally used only in larger engines, primarily V-6 and V-8s with pushrods.