An instrument for writing or drawing with ink or similar fluid, especially: A ballpoint pen. A fountain pen. A pen point. A penholder and its pen point. A quill. An instrument for writing regarded as a means of expression: "Tyranny has no enemy so formidable as the pen\u201d ( William Cobbett). A writer or an author: a hired pen. A style of writing: wrote plays with a witty pen. Pinions. The chitinous internal shell of a squid. To write or compose with or as if with a pen. A fenced enclosure for animals. The animals kept in such an enclosure. Any of various enclosures, such as a bullpen or playpen, used for a variety of purposes. A repair dock for submarines. To confine in or as if in a pen. See Synonyms at enclose. A female swan. Informal A penitentiary; a prison.