"leave"

Two words that immediately come to my mind are "sabbatical" and "moratorium". There is also "leave of absence". Most of these terms have connotations that depend upon the circumstances of the absence, but you may want to put a spin on your explanation to others about why you are dropping out.

The answers Dick provided were what came to my mind too. But it really depends on your connection to the university. Are you a student or a professor? Different terms would apply in each case. I (as a student) had to drop out for one semester just to get a temporary job and earn the tuition for my final semester. When I spoke with the Registrars Office about my intentions, they referred to what I was doing as a "temporary suspension of studies." With my stated intention of returning, that made it easy to get started again and avoided a lot of redundant paperwork.

Sabbatical seems to fit the bill.

Here, in my university, if you apply for the leave, in the announced period before the semester begins, they call it, literally, a 'temporary suspension of studies'. If you do after the semester's start, it's called, literally again, a 'semester omission'.