Apt to intrude; coming unbidden or without welcome; appearing undesirably: as, intrusive thoughts or guests.Done or effected by intrusion; carried out by irregular or unauthorized entrance: as, intrusive interference.Thrust in out of regular place or order; introduced from an extraneous source; due to intrusion or irregular entrance.Specifically, in geology applied to those igneous masses which have forced their way between older rocks and have never reached the surface: contrasted with extrusive. See intrusive rocks, under intrusive.n. In geology, a mass of igneous rock which has forced its way between older walls or strata, but has never reached the surface of the earth. Intrusives occur as batholiths, laccoliths, intruded sheets, and dikes.