knee noun definition in Body parts topic from the Oxford Advanced Learner's Dictionary

knee

 noun: Body parts topic
the joint between the top and bottom parts of the leg where it bends in the middle a knee injury I grazed my knee when I fell. He went down on one knee and asked her to marry him. She was on her knees scrubbing the kitchen floor. Everyone was down on (their) hands and knees(= crawling on the floor) looking for the ring. My knees were knocking (= I was frightened).