A usually soft and close-fitting head covering, either having no brim or with a visor. A special head covering worn to indicate rank, occupation, or membership in a particular group: a cardinal's cap; a sailor's cap. An academic mortarboard. Used especially in the phrase cap and gown. A protective cover or seal, especially one that closes off an end or a tip: a bottle cap; a 35-millimeter lens cap. A crown for covering or sealing a tooth. A tread for a worn pneumatic tire. A fitted covering used to seal a well or large pipe. Chiefly Southern U.S. See eye. A summit or top, as of a mountain. An upper limit; a ceiling: placed a cap on mortgage rates. Architecture The capital of a column. Botany The top part, or pileus, of a mushroom. Botany A calyptra. A percussion cap. A small explosive charge enclosed in paper for use in a toy gun. Any of several sizes of writing paper, such as foolscap. Sports An appearance by a player in an international soccer game, traditionally rewarded with a hat. To cover, protect, or seal with a cap. To award a special cap to as a sign of rank or achievement: capped the new women nurses at graduation. To lie over or on top of; cover: hills capped with snow. To apply the finishing touch to; complete: cap a meal with dessert. To follow with something better; surpass or outdo: capped his last trick with a disappearing act that brought the audience to its feet. To set an upper limit on: decided to cap cost-of-living increases. cap in hand Humbly or submissively. set (one's) cap for To attempt to attract and win as a mate.