intrusion

Short definition from Wiktionary

Noun

1. 1
   The forcible inclusion or entry of an external group or individual; the act of intruding.
2. 2
   Magma forced into other rock formations; the rock formed when such magma solidifies.
3. 3
   A structure that lies within a historic district but is nonhistoric and irrelevant to the district.
4. 4
   The insertion of a phoneme into the pronunciation of a word despite its absence from the spelling. (e.g. intrusive r)
5. 5
   An involuntarily arising idea or memory that is nuisant and falsifies an accurate impression of the world.