audible (adj.)

"able to be heard," 1520s, from French audible and directly from Medieval Latin audibilis "that may be heard," from Latin audire "to hear" (from PIE root *au- "to perceive"). Related: Audibly; audibility; audibleness. As a noun, "thing capable of being heard," from 1610s. Football sense of "signal called at the line of scrimmage to change the play called in the huddle" is by 1958.  

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Definitions of audible from WordNet
audible (n.)
a football play is changed orally after both teams have assumed their positions at the line of scrimmage;
audible (adj.)
heard or perceptible by the ear;
he spoke in an audible whisper
Synonyms: hearable