Consistency (or homogeneity) implies recognizability and often understandability. We like to explain it with the term “(inner) order”, or conceptual integrity.
In 1975, FredBrooks said: “I will contend that ConceptualIntegrity is the most important consideration in system design. It is better to have a system omit certain anomalous features and improvements, but to reflect one set of design ideas, than to have one that contains many good but independent and uncoordinated ideas.” (quote from c2.com)
Consistent systems or models treat identical situations in the same manner, coherent or uniform. They solve similar problems in similar ways.
Consistency in models includes topics like layout, color, naming or uniform usage of symbols.
See also question D-7 (understandability)