 Mathematical structure. In mathematics, a structure on a set is an additional mathematical object that, in some manner, attaches or relates to that set to endow it with some additional meaning or significance. The partial list of possible structures are measures, algebraic structures, groups, fields, etc. topologies, metric structures, geometries, orders, events, equivalence relations, differential structures, and categories. Sometimes, the set is in doubt with more than one structure simultaneously. This enables mathematicians to study it more richly. For example, an ordering imposes a rigid form, shape, or topology on the set. As another example, if a set has both a topology and is a group, and these two structures are related in a certain way, the set becomes a topological group. Mappings between sets which preserve structures so that structures in the source or domain are mapped to equivalent structures in the destination or co-domain are of special interest in many fields of mathematics. Examples are homomorphisms, which preserve algebraic structures, homeomorphisms, which preserve topological structures, and diffeomorphisms, which preserve differential structures.