As opposed to "bottom end" which is all the bits down in the engine block like the crank, con rods and pistons.
Generally refers to the moving parts of the cylinder head.
Basically as Rick said. Anything above the block head mounting surface. Not really a technical term so there is no real technical definition.
THE VALVE TRAIN IS NORMALLY CONSIDERED THE "TOP END.
Cylinder heads, valves, cam and associated parts.
Heads, valves, rocker-arm assembly, push-rods, and lifters.
head's, manifold & carburation.
