Audible describes anything loud enough to be heard and, in American football, names the substitute play a quarterback calls aloud at the line of scrimmage after reading the defense.
As an adjective the word states a capacity rather than an event: an audible signal is one that can be heard, whether or not anyone hears it, which is why engineering specifications pair it with visible to distinguish an alarm's channels. The football sense, in use since the mid-twentieth century, works as both noun and verb — a quarterback calls an audible, or simply audibles. The change is made to exploit a mismatch or escape an unfavorable alignment, and it has to reach ten teammates across a loud stadium in the seconds before the snap, so it is shouted in coded language. The audibility is the mechanism, not a side effect: the play is named for how it must be communicated rather than for the improvisation behind it.
From that specific use the phrase call an audible has generalized well beyond the field, meaning any late change of plan made after seeing the actual conditions — standard business idiom now, and usually a compliment to the person who read the situation in time.
The word came through Middle French from Late Latin audibilis, from Latin audire, "to hear" — the same root that produced audit, audition, and 📝audience, which is literally a body of hearers. An audible is a call rather than a play type; a 📝blitz is one of the defensive looks most likely to provoke one.