英语语法中
preposition（介词） ：用在名词、代词或名词性词组前边,组成介词结构，合起来表示方向、对象等的词。如:从、往、在、当、把、对、同、为、以、比、跟、被等。
介词又称前置词，是英语中最活跃的词类之一，它表示的是它后面的名词或代词(或相当于名词的其他词类、短语或从句)与其他句子成分的关系。
介词可分为下列三类：
1. 简单介词，如：
about, above, across, after, against, among, at, before, behind, below, beneath, beside, besides, beyond, but, by, down, during, except, for, from, in, like, near, of, off, on, over, past, round, since, through, to towards, under, until, till, up, with
2. 合成介词，如：
inside, into, onto, outside, throughout, upon, within, without
3. 短语介词，如：
according to, along with, apart from, as for, as to, because of, by means of, in front of, in spite of, instead, in accordance with, on account of, on behalf of , owing to, due to, together with, up to, with regard to, next to, prior to
介词不能单独充当句子成分，它必须与名词或相当于名词作用的成分构成介词短语才能充当 句子的成分。
参考资料：http://zhidao.baidu.com/