I don't know if there is really one definition of a fragmenting bullet. One example that I can think of is the Glazer Safety Slug. It is a polimartipped bullet that has shot inside the bullet that are sealed in by the tip.It is designed to enter afew inches and and fragment. I believe they were designed for sky marshals. They do not usually exit the body but do devistatingdamage to the internal organs making them safer to use on airplains and crouded situations.