An algebraic structure whose elements consist of selected Homeomorphisms between open subsets of a Space, with the composition of two transformations defined on the largest possible domain. The ``germs'' of the elements of a pseudogroup form a Groupoid (Weinstein 1996).
See also Group, Groupoid, Inverse Semigroup
Weinstein, A. ``Groupoids: Unifying Internal and External Symmetry.'' Not. Amer. Math. Soc. 43, 744-752, 1996.