/ em; NAmE / noun , abbreviation , symbol
■ noun
(also m ) [ C , U ] ( pl. Ms , M's , m's / emz/) the 13th letter of the English alphabet :
'Milk' begins with (an) M / 'M'.
■ abbreviation
1.
(also med. ) (especially for sizes of clothes) medium :
S M and L (= small, medium and large)
2.
(used with a number to show the name of a British motorway ) :
heavy traffic on the M25
■ symbol
(also m ) the number 1 000 in Roman numerals