What does "intrusive" mean?

Intrusive: Apt to intrude; characterized by intrusion; entering without right or welcome. Intrusive rocks (Geol.), rocks which have been forced, while in a plastic or melted state, into the cavities or between the cracks or layers of other rocks. The term is sometimes used as equivalent to plutonic rocks. It is then contrasted with effusive or volcanic rocks. -- In*tru"sive*ly, adv. -- In*tru"sive*ness, n.