M̃ (majuscule: M̃, minuscule: m̃) is a Latin M with a diacritical tilde.
M̃–m̃ is or was used as a grapheme in several languages:
In Italian, the abbreviation Ssm̃o or SSm̃o may be used for Santissimo, "most holy" (similarly Latin Ssm̃us for Sanctissimus).
M̃ and m̃ are used in mathematics and physics, see Tilde#Mathematics and Tilde#Physics
On the Apple ABC extended keyboard, m̃ is made by typing m and then option-shift n.