Oftentimes, flexibility and mobility are interchanged to describe the ability for one to obtain a specific position, stretch, or have an adequate range of motion to be able to go through a particular movement.  There is, however, a difference between mobility and flexibility.  Flexibility, or the ability to be flexible, is known as the muscle, or muscles, synergistic ability to lengthen across a joint, passively.  In simpler terms, it means that if someone laid on their back, and another person stretched their leg for them, being able to easily stretch that leg would be an example of being flexible.  Mobility, on the other hand, is the active ability of a joint to move through a particular range of motion.  Breaking that down means that the muscles need to be flexible enough for the individual to move through the active range of motion that is desired.  For example, if an individual wanted to squat, they need the lower limb musculature, such as the hamstrings and calves, for the sake of brevity, to be flexible enough for the joint to be able to actively articulate the bones into the squat position.  So, if in the same example, the hamstrings and calves are not flexible, then the ability for that individual to be mobile enough to get into the squat would be decreased, and the position would be that of a compromised, or less than ideal, squat position.  All in all, flexibility basically means the muscle’s ability to lengthen, whereas mobility is the joint’s ability to move actively through a range of motion.  One certainly affects the other!