From the ancient Greek ασκησις, askesis, a trial or test: the act of testing something or trying it out. (The term also has various nonwizardly meanings, mostly involved with the art of philosophy: the word is the distant root of such terms and concepts as "asceticism".)

In wizardry, the act of testing a spell to check whether it has the desired effect. The concept of aschesis does not invalidate the received wisdom that "a spell always works"; it does. But it's smart to make sure the spell in question works the way you intended it to.