Flexion means 'bending'.

Knee range of motion.
Flexion range of motion
A straight leg is said to be at zero (0) degrees of flexion. A fully flexed knee may reach about 140 degrees of flexion, depending on the muscle/fat bulk of the limb.
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CLINICAL PUBLICATIONS
Increasing Knee Range of Motion Using a Unique Sustained Method. Shah N. N Am J Sports Phys Ther. 2008 May; 3(2): 110–113.