Full Idea
A 'structure' in model theory has a non-empty set, the 'universe', as domain of variables, a subset for each 'relation', some 'functions', and 'constants'.
Gist of Idea
Model theory 'structures' have a 'universe', some 'relations', some 'functions', and some 'constants'
Source
Robert S. Wolf (A Tour through Mathematical Logic [2005], 5.2)
Book Reference
Wolf,Robert S.: 'A Tour Through Mathematical Logic' [Carus Maths Monographs 2005], p.167