(intransitive) To operate or serve on a submarine. / (transitive) To torpedo; to destroy with a sudden sneak attack. / (intransitive, sometimes figuratively) To sink or submerge oneself.

A submarine. / A submarine sandwich: a sandwich made on a long bun. / (informal) A substitute, often in sports or teaching.

(US, informal) To substitute for. / (US, informal) To work as a substitute teacher, especially in primary and secondary education. / (Britain, informal, soccer) To replace (a player) with a substitute.

Under.

To coat with a layer of adhering material; to planarize by means of such a coating. / (microscopy) To prepare (a slide) with a layer of transparent substance to support and/or fix the sample.

(UK) A commissioned officer of the Royal Navy ranking above a midshipman and below a lieutenant.

Pertaining to the fatty layer under the skin.

The matter or thought presented for consideration in some statement or discussion; that which is made the object of thought or study.

A large landmass which is either smaller than a continent (such as Greenland), or part of an even larger continent (such as the Indian subcontinent).

The Indian subcontinent, South Asia.