Full Idea
If a theory has, up to isomorphism, exactly one model, then it is said to be 'categorical'.
Gist of Idea
A theory is 'categorical' if it has just one model up to isomorphism
Source
Leon Horsten (Philosophy of Mathematics [2007], §5.2)
Book Reference
'Stanford Online Encyclopaedia of Philosophy', ed/tr. Stanford University [plato.stanford.edu], p.23