nth period; nth class period
Counter used in school contexts to specify which period of the day, e.g. 1時限目 (first period), 2時限目 (second period).
See also: 限目
First period is math class.
After third period ends, it's lunch break.
限目 is a more general counter for ordinal periods, while 時限目 specifically refers to class periods in a school timetable.
Compound of 時限 (time period, class period) and 目 (ordinal suffix).