In academia, a fellow is a member of a group of learned people who work together as peers in the pursuit of mutual knowledge or practice. The fellows may include visiting professors, postdoctoral researchers and doctoral researchers.
The title of research fellow is used to denote an academic research position at a university or a similar institution.
Teaching fellow and fellow at a university
The title of teaching fellow is used to denote an academic teaching position at a university or similar institution. The title fellow can be bestowed to an academic member of staff upon retirement who continues to be affiliated to a university in the United Kingdom.
Oxford, Cambridge and Dublin 
At colleges of the universities of Oxford, Cambridge, and Dublin, fellows form the governing body of the college. They may elect a council to handle day-to-day management. All fellows are entitled to certain privileges within their colleges, which may include dining at High Table (free of charge) and possibly the right to a room in college (free of charge).
In Cambridge retired academics may remain fellows. In Oxford upon retirement, a Governing Body fellow would normally be elected a fellow emeritus and would leave the Governing Body. Distinguished old members of the college, or its benefactors and friends, might also be elected 'Honorary Fellow', normally for life; but beyond limited dining rights this is merely an honour. Most Oxford colleges have 'Fellows by Special Election' or 'Supernumerary Fellows', who may be members of the teaching staff, but not necessarily members of the Governing Body.
Graduate school fellowships
In the context of graduate school in the United States and Canada, a fellow is a recipient of a fellowship. Examples are the NSF Graduate Research Fellowship, the National Defense Science and Engineering Graduate Fellowship, the Guggenheim Fellowship, the Rosenthal Fellowship and the Presidential Management Fellowship.
At Harvard and some other universities in the United States, "fellows" are members of the Board of Trustees who hold administrative positions as non-executive trustee rather than academics.
Cambridge and Oxford Colleges
Some senior administrators of a college such as bursars are made fellows, and thereby become members of the governing body, because of their importance to the running of a College.
Teaching fellows in the US
The term used, in the United States, the high school and middle school setting for students or adults that assist a teacher with one or more classes.
Learned or professional societies, or speciality training
Fellows are the highest grade of membership of most professional or learned societies (see for example, the Chartered Institute of Arbitrators or Royal College of Surgeons). 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 a fellowship is acquired varies for each society, but may typically involve some or all of these:
- A qualifying period in a lower grade
- Passing a series of examinations
- Nomination by two existing fellows who know the applicant professionally
- Evidence of continued formal training post-qualification
- Evidence of substantial achievement in the subject area
- Submission of a thesis or portfolio of works which will be examined
Exclusive learned societies such as the Royal Society have Fellow as the only grade of membership, others like the Faculty of Young Musicians (now defunct) have members holding the post of Associate and posts Honoris Causa.
Appointment as an honorary fellow in a learned or professional society can be either to honour exceptional achievement and/or service within the professional domain of the awarding body or to honour contributions related to the domain from someone who is professionally outside of it. Membership of the awarding body may or may not be a requirement.
US medical training
In US medical institutions, a fellow refers to someone who has completed residency training (e.g. in internal medicine, pediatrics, general surgery, etc.) and is currently in a 1 to 3 year subspecialty training program (e.g. cardiology, pediatric nephrology, transplant surgery, etc.).
Industry and corporate
Large corporations in research and development-intensive industries (IBM, Sun Microsystems, Microsoft, Google or Apple in information technology, Bell Labs or L3 Communications in telecommunications, and Boston Scientific in Medical Devices for example) appoint a small number of senior scientists and engineers as fellows. Fellow is the most senior rank or title one can achieve on a technical career, though some fellows also hold business titles such as vice president or chief technology officer.
Nonprofit organizations and government
The title fellow can be used for participants in a professional development program run by a nonprofit or governmental organization. This type of fellowship is a short term work opportunity (1–2 years) for professionals who already possess some level of academic or professional expertise that will serve the nonprofit mission. Fellows are given a stipend as well as professional experience and leadership training.