Teching is any action that allows you to escape the normal consequences of a systematic problem by performing the right action quickly and at the right time.
The name comes from the ‘technique bonus’, or rather, ‘tech. bonus’ that was displayed upon players’ performing these actions, but nowadays the term ‘teching’ is any of a number of classes of actions generally understood to be easy but requiring timing skill (e.g., SFIII parrying displays ‘Tech Bonus’ but does not fall under any class of things called ‘teching’, and is rarely if ever known as such).
As such, these actions are generally understood to be teching: