From Wikipedia

Cognitive synonymy is a property of words or terms distinguished from similarity of mental associations, connotations, emotive responses, and poetic value; it is the information that a word or term expresses such that it is synonymous with a different word's cognitive meaning (as opposed to emotion or mental association elicited). In this sense it is a more precise, technical definition of 'synonymous,' specifically for theoretical (e.g., linguistic and philosophical) purposes.

If a word is cognitively synonymous with another word, they refer to the same thing without regard to other factors. Further, a word is cognitively synonymous to another word if and only if all instances of both words express the same exact thing, and the referents are necessarily identical.

W.V.O. Quine used the concept of cognitive synonymy extensively in his famous paper Two Dogmas of Empiricism where two words were cognitively synonymous if they were interchangeable in every possible instance. For example,

• All bachelors are unmarried men.
• All unmarried men are not married

Quine notes that if we are referring to the word itself, this doesn't apply, as in,

• 'Bachelor' has less than ten letters.

As compared to the substitution which is obviously false,

• 'Unmarried male' has less than ten letters.

See also

• Synonym
• Synonym ring
• Quotation
• Analytic-synthetic distinction
• Mental association
• a priori
• W.V.O. Quine
• Empiricism
• Two Dogmas of Empiricism

References

• Cognitive Synonymy from Notes on "Two Dogmas of Empiricism", Patricia Hanna, University of Utah
• 1951, "Two Dogmas of Empiricism," The Philosophical Review 60: 20-43. Reprinted in his 1953 From a Logical Point of View. Harvard University Press.
• Cappelen, Herman and Ernest LePore, Quotation, The Stanford Encyclopedia of Philosophy (Winter 2008 Edition), Edward N. Zalta (ed.).