细节(xì jié) - detail, particulars in Mandarin

细节 (xì jié) is the small print of anything: the particulars of a plan, the fine points of a story, the bits of a design that only show up close. It is a countable noun, so 一个细节 and 很多细节 are both normal.

It sits in business and journalism far more than in casual chat. Over tea you would say 你说清楚一点; in a meeting you say 请说明细节. The stock collocations are worth learning as blocks: 注意细节 (mind the details), 细节决定成败 (the details decide the outcome), and 具体细节 for the concrete particulars.

The English speaker's trap is the phrase "in detail". That is not 在细节, it is the adverb 详细 or 仔细: 请详细说明. Keep 细节 for the things themselves and use a different word when you mean the manner of describing them.