Definition 0.1   A variable groupoid is defined as a family of groupoids indexed by a parameter $\lambda \in T$ , with $T$ being either an index set or a class (which may be a time parameter, for time-dependent or dynamic groupoids). If $\lambda$ belongs to a set $M$ , then we may consider simply a projection , which is an example of a trivial fibration. More generally, one can consider a fibration of groupoids (Higgins and Mackenzie, 1990) as defining a non-trivial variable groupoid.

Remarks An indexed family or class of topological groupoids with $i \in I$ in the category Grpd of groupoids with additional axioms, rules, or properties of the underlying topological groupoids, that specify an indexed family of topological groupoid homomorphisms for each variable groupoid structure.

Besides systems modelled in terms of a fibration of groupoids, one may consider a multiple groupoid defined as a set of $N$ groupoid structures, any distinct pair of which satisfy an interchange law which can be formulated as follows. There exists a unique expression with the following content:

\begin{equation} \begin{bmatrix} x&y \\z&w \end{bmatrix}\quad \directs{j}{i}, \end{equation} where $i$ and $j$ must be distinct for this concept to be well defined. This uniqueness can also be represented by the equation \begin{equation} (x\circ_j y)\circ_i(z \circ_j w)= (x\circ_i z)\circ_j(y \circ_i w). \end{equation} Remarks This illustrates the principle that a 2-dimensional formula may be more comprehensible than a linear one.

Brown and Higgins, 1981a, showed that certain multiple groupoids equipped with an extra structure called connections were equivalent to another structure called a crossed complex which had already occurred in homotopy theory. such as double, or multiple groupoids (Brown, 2004; 2005). For example, the notion of an atlas of structures should, in principle, apply to a lot of interesting, topological and/or algebraic, structures: groupoids, multiple groupoids, Heyting algebras, $n$ -valued logic algebras and $C^*$ -convolution -algebras. Such examples occur frequently in Higher Dimensional Algebra (HDA).