In anatomy, flexion (from the Latin word flectere, to bend) is a position that is made possible by the joint angle decreasing. The skeletal (bones, cartilage, and ligaments) and muscular (muscles and tendons) systems work together to move the joint into a "flexed" position. For example the elbow is flexed when the hand is brought closer to the shoulder. The trunk may be flexed toward the legs or the neck to the chest.
The opposite term is extension, or straightening. Flexion decreases the angle between the bones of the limb at a joint, and extension increases it.
Note that specific flexion activities may occur only along the sagittal plane, i.e. from the forward to backward direction, and not side-to-side direction, which is further discussed in abduction.
Read more about Flexion: Exercises