When you blow off studying for a final in college because winter break is only a week away. Symptoms include but are not limited to: Not studying, going to basketball games, going out on the weekend, procrastinating, or being on Facebook for hours.
Very similar to high school Senioritis, except this type of illness plays an important role in decreasing your chances of getting into Grad school, internships, jobs etc.
Person 1: How did you do on the Government final?
Person 2: I probably failed. I came down with a case of Semesteritis so I partied all weekend.