Mental reservation
Black's Law Dictionary · Henry Campbell Black, M.A. · 1910
Black's Law Dictionary
A silent exception to the general words of a promise or agreement not expressed, on account of a general understanding on the subject. But the word has been applied to an exception existing in the mind of the one party only, and has been degraded to signify a dishonest excuse for evading or infringing a promise.
Wharton.