n. The act of intruding; the act of entering without warrant or justification; unbidden, unwelcome, or unfit entrance into or upon anything.n. Specifically, in law: A wrongful entry after the determination of a particular estate, say for life, and before the freehold remainderman or reversioner can enter.n. In English law, any trespass committed on the public lands of the crown, as by entering thereon without title, holding over after a lease is determined, taking the profits, cutting down timber, and the like.n. Usurpation, as of an office.—n. A thrusting or pushing in, as of something out of place; irregular or abnormal entrance or irruption: as, an intrusion of foreign matter; the intrusion of extrinsic rocks or dikes in a geological formation. See intrusive rocks, under intrusive.