Mem
From BiblePortal Wikipedia
Hastings' Dictionary of the Bible [1]
MEM. The thirteenth letter of the Hebrew alphabet, and as such employed in the 119th Psalm to designate the 13th part, each verse of which begins with this letter.
Holman Bible Dictionary [2]
Psalm 119:97-104
International Standard Bible Encyclopedia [3]
mām , mem (, מ ) ם : 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.