The degree of uniformity, standardization, and freedom from contradiction among the documents or parts of a component or system.

IEEE Standard 610.12-1990 IEEE Standard Glossary of Software Engineering Terminology [IEEEXplore Digital Library]

Agreement or harmony of parts or features to one another or a whole: correspondence; specifically: ability to be asserted together without contradiction.

Dictionary Merriam Webster [Dictionary Merriam Webster]

Two design models are consistent if and only there are no contradictions in the viewpoint described in them.

Adapted from Dissertation Tudorache, Tania. "Employing ontologies for an improved development process in collaborative engineering." Fakultät IV–Elektrotechnik und Informatik. Berlin: Technische Universität Berlin (2006). [Dissertation]