Flexion means 'bending'.
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.
|0 degrees/straight leg||90 degrees/mid flexion||143 degrees/full flexion|
When a person has a 'flexion deformity' or 'flexion contracture' it means that they cannot fully straighten their leg. There may, in addition, be a limitation of flexion, meaning that they also cannot fully bend their leg.