信号

In daily conversation, 信号 is almost always used as a shorthand for 信号機 (shingōki), referring to traffic lights. When talking about traffic, it is common to pair the word with colors, such as 青信号 (ao-shingō) for green and 赤信号 (aka-shingō) for red. Note that in Japanese, the "green" light is referred to as "blue" (ao), a linguistic remnant from when the word "ao" covered both blue and green spectrums.

Beyond traffic, the word refers to any signal used for communication, ranging from a simple hand gesture or a flare to complex electrical impulses in telecommunications or neural signals in biology. In these contexts, it functions as a technical term for "signal" or "impulse."

Metaphorically, 信号 can be used to indicate a sign or a cue that something is about to happen or that permission has been granted. For example, receiving a "green light" (青信号) can figuratively mean getting the go-ahead for a project or plan, similar to the English idiom.