What is considered full-time enrollment for 2016, versus "at least half-time"

This, from the IRS: 

Full-time student .   You are a full-time student for 2016 if during any part of any 5 calendar months during the year you were enrolled as a full-time student at an eligible educational institution (defined earlier), or took a full-time, on-farm training course given by such an institution or by a state, county, or local government agency. 

The definition of "full-time student" varies from place to place.  Check with the institution that you attended to see what their rules are.