A person who enters a place without permission, often forcefully or disruptively. Used in contexts like unauthorized entry into events, private property, or secure areas.
An intruder appeared at the wedding, causing a commotion.
The security guard subdued the intruder.
Compound of 乱入 (ran'nyū, 'forcible entry') + 者 (sha, 'person'). The exact historical derivation is uncertain, but the word follows a common pattern for forming agent nouns.