in a word 为习语，非正式用法，用于简单回答别人。《牛津》的解释如下：

in a word：(informal) used for giving a very short, usually negative, answer or comment

‘Would you like to help us?’ ‘In a word, no.’

in one word 通常用来概括总结前面说过的内容，可翻译为：一句话/总而言之。

In one word, we must try our best to do the job.
总而言之，我们必须尽力去做这项工作。