Having the ability to change, especially used in the context of having the ability to change easily. Such as in dynamic HTML, which is HTML that changes it's content and/or layout without reconnecting to the web server.

Also used in reference to friction that counters a constant energy source. Dynamic friction is why we need to continue to supply energy to our cars after they have already reached the desired velocity; As opposed to static friction, where energy is supplied only once per instance to put an object into initial motion.