Most often a cabinet knob, but also a door handle knob. Usually cast glass which may then be facetted. Common pull knobs have a hole through them and are mounted with a bolt, the head showing. Emtek sells knobs that are mounted in a brass base that takes a bolt from behind. Getting bases is not easy. Could be produced as small paperweights, glued or crimped into a base, or as specially shaped beads.
'Playing up to the knob'. Usually, but not necessarily, the crown of a green. It could be any high point on the green, and is used to good advantage by some players who specialise in short marks where delicate and accurate delivery is required.