An intrusion is a deliberate move into someone else's territory — either literal or figurative. When your sister interrupts your conversation with that girl from math class, that's an intrusion. If someone breaks into your home, that's also an intrusion.
First used in the late 14th century, the noun intrusion derives from the Latin word intrudere, which combines the prefix in-, meaning "in," and trudere, meaning "to thrust, push." If someone reads your diary, that's considered an intrusion of privacy. Ordering a Muslim woman to take off her veil would be considered an intrusion on religious beliefs. You may remember intrusion used in science class to describe molten rock that forms in an earlier rock formation.