1. A disturbance of someone's peace or privacy by an unwelcome arrival or presence: The ringing of the alarm clock at 6:30 in the morning was an intrusion into Jason's pleasant dream.
2. The action of thrusting in, or of entering into a place or situation without an invitation, a right, or welcome: The birthday party was disturbed by the intrusion of an uninvited guest.
3. An unwelcome presence or effect that agitates, unsettles or upsets something: When Kitty was staying in the hotel that her friends had recommended, she was bothered by the intrusion of the maid who entered her room without knocking.
4. An illegal entry into a place, often by force, in order to commit a crime; When the alleged burglar appeared in court, he was charged with intrusion with the intent to commit a robbery of the elderly couple who lived in the house.
5. A body of melted igneous matter that has moved while in the form of magma into pre-existing rocks with subsequent alterations of those elements: The geologist and students were interested in studying the pattern of intrusions, or flows, of lava into the cracks of earlier volcanic material on the same hillside.