We call it annual leave when it comes to taking time off from work or school for a year. I want to use something like "Semester leave" as the title of a letter to my university to denote that I will be taking a semester off. Are there any better alternatives to "Semester leave"? Note that I'm looking for a title, so please don't suggest long phrases or complete sentences.
You could just use the term semester off or time off
time for rest or recreation away from one's usual work or studies.
"we're too busy to take time off"
If I was writing an email I would probably title it something like "Taking the Semester Off" or similar.