A word, which can be put before a noun or a pronoun to form a prepositional phrase indicating time, place, direction, object, reason, manner, the passive, comparison, or exclusion, etc. is called a preposition.

• 用在词或词组前头，合起来表示时间、方向、处所、方式、方法、手段、原因、目的、对象、关联、比较或排除等的词叫介词。