nth period; counter for school periods
Used as a suffix after numbers to indicate which period of a school timetable, e.g. 1限 (first period), 2限 (second period).
See also: 時限
First period is math class.
There's a math class during 3rd period.
I only have classes up to third period today.
Today's classes end after 4th period.
時限 is the more formal, written term for a class period, while 限 is the casual counter suffix attached directly to numbers (2限) in everyday student speech.
限目 adds 目 to mark the ordinal explicitly (2限目), while 限 alone (2限) is the shorter, more common way students refer to the same period.
Derived from the kanji 限 (limit, bound), used here as a counter for bounded time slots in a school schedule.