A locum is a person who temporarily fulfills the duties of another. For example, a locum physician is a physician who works in the place of the regular physician when that physician is absent, or when a hospital/practice is short-staffed. These professionals are still governed by their respective regulatory bodies, despite the transient nature of their positions.
The word locum is short for the Latin phrase locum tenens ("place holder", akin to the French lieutenant). The abbreviated form "locum" is common in Australia, Canada, Ireland, Malaysia, Singapore, New Zealand, South Africa and the United Kingdom; unlike in Latin its plural is locums. In the United States, the full length "locum tenens" (plural: locum tenentes) is preferred, though for some particular roles, alternative expressions (e.g., "substitute teacher") may be more commonly used.