a letter from an official source expressing a command, permission, invitation, etc.
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How to use letter missive in a sentence
This is exactly what would happen now, if the letter missive should be lost on the road and the cong d'lire should come by itself.History of the Cathedral Church of Wells | Edward A. Freeman
But the terms of the letter missive were so vague that, though it bore the sign manual, Casaubon hesitated to act upon it.