A fellow in the broadest sense is someone who is an equal or a comrade. The term is most often used in an academic context: a fellow is (at least in theory) part of an elite group of learned people who work together as peers in the pursuit of knowledge. However, there are no precise rules for how the title is used, and each academic institution grants the title as it sees fit.
The term "Fellow" is most often used to describe the incumbent of a temporary academic post. Generally, a fellow of this type has very limited teaching duties and devotes the bulk of her time to research.
A junior research fellow is known informally as a "postdoc". The position is equivalent to research associate, but whereas a research associate works on a specific project under the direction of a supervisor, a research fellow has greater freedom to work independently and choose her own projects. A research associate's salary is paid out of her supervisor's budget, whereas a research fellow is paid either from central university funds or by an outside organisation such as a charity or company.
A senior research fellow is an established academic, often a Professor on sabbatical from another institution. The research fellow may be granted the title of Professor by her temporary employer; if she is, she is more likely to be referred to as a Research Professor.
The Colleges of the Universities of Oxford, Cambridge and Trinity use the term "fellow" in a more specific sense. The fellows of a College form the governing body of the College, although they may elect a Council to handle day-to-day management. All fellows are entitled to certain privileges within their College, such as free meals at High Table and the right to a rent-free room in College.
Broadly speaking, College fellows fall into five groups:
Except for research fellows, College fellows have no duties as such and are not paid. They will typically have a salaried post either with their College or the University.
The term "praelector" is used for some types of fellow.
Fellows are the highest grade of membership of most professional societies. Lower grades are referred to as members (who typically share voting rights with the fellows), or associates (who may or may not, depending on whether "associate" status is a form of full membership).
How fellowship is acquired varies from society to society. It is typically involves some or all of:
Fellowships include:
Historically, the word "fellow" was also used to describe a man, particularly by those in the upper social classes.
In Christianity, fellowship means individual Christians who are in communion with each other. Today the word fellowship is often used to designate a group, either run by a denomination or is interdenominational, aimed at fostering a sense of fellowship (that is, mutual support and common purpose) among Christians of a particular demographic. An example of a large fellowship would be the InterVarsity Christian Fellowship.
In the USA, a period of medical training following on from residency is sometimes called fellowship.
In medical education, a fellow is generally a medical doctor who has finished a residency and is studying a specialty.
Alcoholics Anonymous considers itself a fellowship in the sense that it has no hierarchy or other organizational structure, and hence all members are equals.
The Fellowship of the Ring, a fellowship of nine different fictional characters (4 hobbits, 2 men, a wizard, elf and a dwarf) featured prominently in JRR Tolkien's Lord of the Rings series.