To circumscribe is to limit or restrict. If you spend too much time watching TV instead of fulfilling other obligations, you might circumscribe your TV-watching time to one hour daily (or two hours, if your favorite show is on).
Circumscribe comes from the Latin words circum, meaning "around," and scribere, meaning "to write." If the word circumscribe causes you to think of geometry class, give yourself a pat on the back. In geometry, circumscribe means to draw one geometric figure around another figure so that the two figures touch but don’t intersect. If you studied geometry but this sounds unfamiliar, perhaps you should have taken our advice about watching too much TV.