an action, movement, or sound that gives information, a message, a warning, or an order

When she gave (them) the signal, they all cheered. 她发出信号后，他们都欢呼起来。

[ + that ] The fireworks were a signal that the festival had started. 焰火是节日开始的信号。

[ + to infinitive ] The police officer gave us the signal to stop. 警察示意我们停下。

The signal for a race to start is often the firing of a gun. 赛跑常以发令枪响作为开始的信号。

a turn signal

a series of electrical or radio waves that are sent to a radio or television in order to produce a sound, picture, or message

something that shows that something else exists or is likely to happen

The loss is a clear signal of his deteriorating confidence. 比赛输了，这清楚地表明他信心日渐不足。

The changing colour of the leaves on the trees is a signal that it will soon be autumn. 树叶颜色的改变表明秋天即将来临。

equipment, especially on the side of a railway or road, often with lights, that tells drivers to stop, continue, or go more slowly

a railway signal 铁路信号灯

a traffic signal 交通信号灯

a road signal 道路信号灯

to make a movement, sound, flash, etc. that gives information or tells people what to do

Flashing lights on a parked car usually signal a warning (to other drivers). 停着的车辆上闪烁的车灯通常是警告（其他驾车者）的信号。

He signalled left, and turned the lorry slowly. 他打开左转方向灯，然后将卡车缓慢转过去。

He was signalling (= giving a signal) with a red flag. 他正用红色旗子发信号。

She signalled for help. 她发出了求救信号。

[ + that ] She signalled to the cars behind her that they were going the wrong way. 她示意后面的车辆走错了路。

[ + obj + to infinitive ] The children's mother signalled them to be quiet. 孩子们的母亲示意他们安静。

[ + to infinitive ] The children's mother signalled to/for them to be quiet. 孩子们的母亲向他们示意保持安静。

to show that you intend or are ready to do something

[ + that ] The union has signalled that the workers will strike. 工会表示工人们将会罢工。

The union has signalled the workers' intention to strike. 工会已表示工人们打算罢工。

The death of Chairman Mao signalled (= marked) the end of an era in Chinese history. 毛主席的去世标志着中国历史上一个时代的结束。

noticeable and unusual

a signal success/failure 重大胜利／失败