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a. The joint between the thigh and the lower leg, formed by the articulation of the femur and the tibia and covered anteriorly by the patella.
b. The region of the leg that encloses and supports this joint.
2. An analogous joint or part of a leg of a quadruped vertebrate.
3. The joint between the femur and the tibia in an insect leg.
4. Something resembling the human knee, such as a bent piece of pipe.
5. The part of a garment, as of trousers, that covers the knee.
6. A vertical, often conical, woody projection arising from the roots of certain swamp-growing trees: cypress knees.
tr.v. kneed, knee·ing, knees
To strike with the knee.
take a knee
1. To kneel down on one knee.
2. Football To kneel down on one knee while holding the ball so as to down the ball, as in one's own end zone for a touchback.

[Middle English, from Old English cnēo; see genu- in Indo-European roots.]