n. the extension of a joint beyond its normal range; the condition of being hyperextended
The Century Dictionary and Cyclopedia
n. The extension of a joint beyond the straight line or the normal limit of such movement.
WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
n. greater than normal extension
Word Usage
"A hyperextension is the same as a sprain - "hyperextension" refers to the mechanism of the injury - bending the joint in a direction that it isn't intended to go, usually due to a hard fall."