extensor

[New Latin extēnsor, from Latin extēnsus, past participle of extendere, to stretch out. See extend.]

A muscle which straightens a joint.

A muscle that extends or straightens a limb or body part.

[L.] any muscle that extends a joint.for list of all muscles.

• crossed e. reflex — see crossed extensor reflex.
• e. postural thrust — a postural reflex reaction tested in small animals by lifting the patient off the ground, then lowering it to see whether the hindlegs are extended to make contact and support weight and several short steps are taken to maintain posture. Called also extensor reaction.

  Extensor postural thrust response. By permission from Sharp NJH, Small Animal Spinal Disorders, Mosby, 2004

• e. process disease — see pyramidal disease.
• e. reaction — see extensor postural thrust.
• e. rigidity — see under upper motor neuron.

The noun has one meaning:

Meaning #1: a skeletal muscle whose contraction extends or stretches a body part
  Synonym: extensor muscle
  Antonym: flexor (meaning #1)