A basic unit used to record system activity. The acceptance by a system of a line of input from a terminal, processing of the line, and a return of data, if any, to the terminal.

• 用于记录系统活动的一个基本单位，即人－机对话的过程。系统由终端接收一行输入进行处理，如果有结果信息，再送回到终端。