A fixed idea or bias formed before knowing the facts; often used in the phrase 先入主にとらわれる (to be bound by preconceptions).
Let's listen to the story first without being bound by preconceptions.
His opinion is based on prejudice.
先入観 is a near-synonym also meaning preconception, but 先入主 is less common and may carry a slightly stronger nuance of a fixed, subjective viewpoint.
Compound of 先入 (sennyū, 'entering beforehand') and 主 (shu, 'master, main'). The exact historical derivation is uncertain, but the word conveys the idea of a preconceived notion that dominates one's thinking.