first shift

Short definition from Wiktionary

Noun

1. 1
   A regularly scheduled period of work, being the first one in the standard working day of any particular company: usually during the day; often the first of three (with 24/3 = 8 hours long), and often from 8:00 a.m. to 4:00 p.m.; sometimes the first of two, and sometimes with other time values.
2. 2
   A group of workers who work during this time slot.