piriformis
noun
pir·i·for·mis
variants:
or pyriformis \ ˌpir-ə-ˈfȯr-məs 
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Medical Definition of piriformis
: a muscle that arises from the front of the sacrum, passes out of the pelvis through the greater sciatic foramen, is inserted into the upper border of the greater trochanter of the femur, and rotates the thigh laterally