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Monoids abound in mathematics; they are in a sense the most primitive interesting algebraic structures.http://math.ucr.edu/home/baez/week74.html
what is a category with one object? It is a "monoid". The usual definition of a monoid is this: a set M with an associative binary product and a unit element 1 such that a1 = 1a = a for all a in S. http://math.ucr.edu/home/baez/week74.html