noun
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.
結婚式に乱入者が現れて、会場が騒然となった。
An intruder appeared at the wedding, causing a commotion.
警備員が乱入者を取り押さえた。
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.