事先 (shìxiān) is a noun meaning 'in advance, beforehand', used to refer to the time before an event happens. It is more formal than the adverb 先 (xiān 'first, ahead of others'), which focuses on order rather than preparation. Unlike the verb 提前 (tíqián 'to move forward'), 事先 does not imply changing a schedule — it simply means 'prior to the event'.