fellow

adj. having membership in, belonging to (a group, academic organization, etc.); of the same type

n. guy, chap; man or boy; friend, buddy; suitor; spouse; colleague; member of an academic organization

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.
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Noun
1. a boy or man; "that chap is your host"; "there's a fellow at the door"; "he's a likable cuss"
(synonym) chap, feller, lad, gent, fella, blighter, cuss
(hypernym) male, male person
(hyponym) dog
2. a person who is frequently in the company of another; "drinking companions"; "comrades in arms"
(synonym) companion, comrade, familiar, associate
(hypernym) friend
(hyponym) date, escort
3. a person who is member of your class or profession; "the surgeon consulted his colleagues"; "he sent e-mail to his fellow hackers"
(synonym) colleague, confrere
(hypernym) associate
4. an informal form of address for a man; "Say, fellow, what are you doing?"; "Hey buster, what's up?"
(synonym) buster
(hypernym) man, adult male
5. a man who is the lover of a girl or young woman; "if I'd known he was her boyfriend I wouldn't have asked"
(synonym) boyfriend, beau, swain, young man
(hypernym) man, adult male