in another word，是有人根据 in other words 发明的一个词组。一个 word 只是一个单词，只有 words 才可以表示一句话。所以，“换句话说”，应该是后者，才是规范的说法。

N-COUNT A word is a single unit of language that can be represented in writing or speech. In English, a word has a space on either side of it when it is written. 单词
N-PLURAL Someone's words are what they say or write. 话