Usually it refers to an exam which you write down words or passages even a story that the teacher reads out, you just write down what is being read out by the teacher or examiner.
n.
1. 口述;聽寫[U]
The pupils wrote at their teacher's dictation.
學生按照老師的口述寫下來。
2. 口授(或聽寫)的文字[C]
3. 命令[U]
She was tired of her husband's dictation.
她對丈夫的發號施令感到厭煩。
Usually it refers to an exam which you write down words or passages even a story that the teacher reads out, you just write down what is being read out by the teacher or examiner.