alien; extraterrestrial
Refers to an intelligent being from another planet. More formal and less common than 宇宙人, but carries a similar meaning.
That movie depicts an encounter with aliens.
Some scientists believe in the existence of extraterrestrials.
地球外生命体 is a scientific term for 'extraterrestrial life form', broader than 異星人 which implies a humanoid or intelligent being.
Compound of 異 (different) + 星 (planet/star) + 人 (person). The exact origin is uncertain, but it follows the pattern of words like 外国人 (foreigner) and 宇宙人 (space person).