What is muscle stretching?

The answer to this question is not so easy as you may think. 

Many people assume that the term "stretching" and "exercise" are the same thing.  They're not; in fact, in some ways they can be considered opposites. 

Muscle exercises consist of a muscle tightening and contracting (or shortening) under some degree of resistance.  The process of exercise strengthens muscles.

Muscle stretching, however, is quite different.  In a stretch, the muscle is relaxed (not contracted) while it undergoes gentle elongation.  It's important to emphasize that the muscle must be relaxed to stretch--if it's not relaxed, but is tight and contracting, the muscle cannot, by definition, stretch. 

What is the purpose of stretching a muscle? 

There are several purposes for stretching.  First, stretching allows for more effective muscle contraction during exercise, thereby increasing power.   Second, stretching may allow certain joints to have a better range of motion.  Stretching also diminishes odds of injury and can slow down degenerative processes throughout the musculoskeletal system.