In 4-stroke IC engines, usually a poppet valve, located in the cylinder head, operated directly or indirectly by a camshaft; the inlet valve is usually larger and (on used engines) cleaner than the exhaust valve; it is only open during the induction stroke.
Dealying the closure of the inlet valve until after BDC, when the piston has commenced its compression stroke, improves the cylinder volumetric efficiency in the medium to high engine-speed range.