定(dìng) - to settle, to fix, to arrange in Mandarin

定 (dìng) is the settling morpheme of Mandarin. On its own it means to fix or to arrange: 时间定了 means the time is agreed, 价格定好了 means the price is set, and 就这么定了 is the standard way of closing a discussion.

Most of the time you meet it inside a compound, and the compounds are everywhere: 决定 to decide, 一定 certainly, 肯定 definitely, 确定 to confirm, 规定 a regulation, 稳定 stable. If you can see 定 in a new word, you can usually guess that the word has something to do with fixing, settling or being sure.

Two traps. First, bare 定 is terse. In conversation people reach for the compound, saying 我们决定明天走 rather than 我们定明天走, and 定下来 is the natural spoken way to say something has been settled. Second, for booking a ticket or a table, the more careful written character is 订 (dìng), with the speech radical: 订机票, 订房间. The two sound identical and are often swapped in casual writing, but 订 is the one an editor will want.