The sound made when jets, etc., cross the speed of sound.
From Sonic X (Japanese version with English subtitles (fan sub)):
Sam Speed (when he begins using the "Super Nitro"): Super Nitro - ON!
Sam Speed (while using the "Super Nitro"): Out here, I can't accept that someone is faster than me. If you want to pass me, you'll have to break the sound barrier!
*Sonic begins to create a sonic boom while passing Sam Speed*
Sam Speed: Sonic Boom!?
*This exact definition shows up in the anime, see the picture of it with the large yellow SONIC BOOM words above*
Sam Speed: It's over.
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