A bellow is the name for the sound a bull makes. Bulls who bellow do so under duress and with hostility. Kicking, butting, walking the fence, and adopting varied postures are additional examples of hostility.
One should never turn away from a bull. The person should move carefully if they are within the flight zone, which is roughly 20 feet. Any quick movement draws a pursuit. The best form of self-defense is to face the bull and gently back away to safety. In rare cases, if the opponent approaches from a distance of 20 feet or more, a bull making a threat display will retire and turn away.