MEM mām (ם). The thirteenth letter of the Heb. alphabet. It is used to introduce the thirteenth part of the acrostic psalm, Psalm 119, in which section every v. begins with this letter. It also was used for the number 40.

International Standard Bible Encyclopedia (1915)

mam, mem "m" : The 13th letter of the Hebrew alphabet, transliterated in this Encyclopedia as "m". It came also to be used for the number 40.

See Alphabet, for name, etc.