noun
intruder; infiltrator
Refers to a person who secretly enters a place, organization, or system, often with a specific purpose such as spying, sabotage, or unauthorized access.
警備員は建物に潜入者を見つけた。
The security guard found an intruder in the building.
彼は敵組織に潜入者として送り込まれた。
He was sent into the enemy organization as an infiltrator.
スパイ is a loanword for 'spy', focusing on espionage, whereas 潜入者 can be any kind of infiltrator, not necessarily a spy.
Compound of 潜入 (せんにゅう, 'infiltration') + 者 (しゃ, 'person'). The word is transparently derived from its components.