Locum, short for the Latin phrase locum tenens (lit. "place holder," akin to the French lieutenant), is a person who temporarily fulfills the duties of another. For example, a locum doctor is a doctor who works in the place of the regular doctor when that doctor 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 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.
In the UK, the NHS on average has 3,500 locum doctors working in hospitals on any given day, with another 15,500 locum general practitioners. Many of these locum hospital doctors are supplied by private agencies through a national framework agreement that the NHS holds with 51 private agencies. NHS figures show that approximately 80% of hospital locum positions are filled by agencies on this framework. The remaining 20% are filled by agencies working outside of this agreement. Locum agencies are common reference points for doctors wishing to work in this market. According to a report published by Royal College of Surgeons NHS spent approximately £467 million on locum doctors through agencies in the year 2009-2010.
On the other hand, GP locums (freelance GPs) mostly work independently (c.75%) from locum agencies either as self-employed or via freelance GP chambers based on the NASGP's Sessional GP Support Team (SGPST) model. Some GPs in the UK have been employed by the Primary Care Trusts (PCTs) to provide locum cover. However, PCTS are now facing closure under the Government's health plans and their services have been discontinued. Locum GPs are expected to be flexible, adaptable, resourceful, professional, quick to establish relationships, familiar with different IT systems, and able to independently manage risks.
Locum tenens is also a name given to a temporary hierarch in the Eastern Orthodox Church, e.g., locum tenens of the Russian Orthodox Church.
Advantages and disadvantages 
Locums provide a ready means for organizations to fill positions that are either only temporarily empty (during sickness, leave or for other reasons) or for which no long-term funding is available. Locumming also allows a professional to try (and get experienced in) a wide range of work environments or specialisation fields which a permanent employee may not encounter. Locum tenens assignments allow you to choose when and where you practice medicine and which job opportunities fit your immediate and long-term needs. 
However, the locum situation also has a number of disadvantages – the transient nature means extra stress and work for the locums whenever they have to fit into a new position, and for the hiring organisation, this generally means that the required flexibility (and often, the lack of a guaranteed income) has to be rewarded with high salaries. These may in the long term create higher costs for the hiring organisation than adding more full-time positions (especially in highly-skilled, accredited professions, and unlike the situation in some professions where cheap temporary labour or significant use of interns actually undercuts wages and reduces total staff costs).
Also, and especially true in professions where knowing all procedures and past case histories is important (such as for doctors working on patients, who may dislike not being treated by their own doctor, or by constantly shifting doctors), locums may provide lesser-quality work (or be seen as posing such a risk, fairly or not). Further, locums often experience resentment from permanent staff, for example because they are paid more, or because they are considered to shoulder less responsibility.
Common uses 
Professions where locums are common include:
|Look up locum in Wiktionary, the free dictionary.|
- "NHS locums need 'checks and training'". BBC News. 1999-04-21. Retrieved 2010-01-20.
- "Salaried GPs made redundant as PCTs 'shut up shop'". Pulse Today. 18 November 2011
- Career focus - Locum doctors - Dr Philip Morgan, BMJ 7074 Volume 314: Saturday 11 January 1997
- Locum pharmacy - Pamela Mason, 2004, Page 11