When used to refer to a job, per diem refers to casual employment situations where people are hired for one of two days at a time. Per diem is a Latin expression and translates to, "by the day." It is used primarily in the health care industry.
Per-diem employees are most often hired to fill in for permanent staff members who are sick or on vacation. A hospital, for instance, may have a list of per-diem nurses who can be asked at short notice to fill a vacancy in any department.
This term is also used to refer to a set amount of money paid out per day to staff or volunteers to cover employment-related expenses like meals and gas when traveling for business.