Given a groupoid ${\displaystyle (G,\cdot )}$  (in the sense of a category with all morphisms invertible) and a field ${\displaystyle K}$ , it is possible to define the groupoid algebra ${\displaystyle KG}$  as the algebra over ${\displaystyle K}$  formed by the vector space having the elements of (the morphisms of) ${\displaystyle G}$  as generators and having the multiplication of these elements defined by ${\displaystyle g*h=g\cdot h}$ , whenever this product is defined, and ${\displaystyle g*h=0}$  otherwise. The product is then extended by linearity.^[2]

Some examples of groupoid algebras are the following:^[3]

• Group rings
• Matrix algebras
• Algebras of functions

• When a groupoid has a finite number of objects and a finite number of morphisms, the groupoid algebra is a direct sum of tensor products of group algebras and matrix algebras.^[4]

• Hopf algebra
• Partial group algebra

• Khalkhali, Masoud (2009). Basic Noncommutative Geometry. EMS Series of Lectures in Mathematics. European Mathematical Society. ISBN 978-3-03719-061-6.
• da Silva, Ana Cannas; Weinstein, Alan (1999). Geometric models for noncommutative algebras. Berkeley mathematics lecture notes. Vol. 10 (2 ed.). AMS Bookstore. ISBN 978-0-8218-0952-5.
• Dokuchaev, M.; Exel, R.; Piccione, P. (2000). "Partial Representations and Partial Group Algebras". Journal of Algebra. 226. Elsevier: 505–532. arXiv:math/9903129. doi:10.1006/jabr.1999.8204. ISSN 0021-8693. S2CID 14622598.
• Khalkhali, Masoud; Marcolli, Matilde (2008). An invitation to noncommutative geometry. World Scientific. ISBN 978-981-270-616-4.