what are intrusions
noun. The definition of an intrusion is an unwelcome interruption or a situation where somewhere private has an unwelcome visit or addition. When you are having a quiet nap in your backyard and your neighbor's dog comes in uninvited and jumps all over you to wake you up, this is an example of an intrusion.oun. (plural intrusions). 1  The forcible inclusion or entry of an external group or individual; the act of intruding. ( 2 geology) Magma forced into other rock formations; the rock formed when such magma solidifies.