signal 
英 [ˈsɪɡnəl]  美 [ˈsɪgnəl] 
n. 暗号；预兆；信号；导火线

v. 发信号；用信号通知，示意；标志

例句：
He seemed to be signalling, but I didn't understand what he wanted me to do. 

他似乎在向我示意，但我不明白他想让我做什么。

He signalled for the bill. 

他示意结账。

If you're going to turn you're supposed to signal. 

如果要转弯，应该打转向灯示意。

He waited for the signal to speak. 

他等着发言的信号。

As soon as it was dark, she gave the signal. 

天一黑，她就发出了信号。