A Hebrew name for Moses Maimonides, sometimes seen in English transliteration too.

Vowels in Hebrew are secondary, so an abbreviation can be (and is) made from the consonants alone, and any old vowels (usually a) are used to fill in the word so it can be pronounced. So Rabbi Moses ben Maimon goes to R-M-B-M, and pronounced "Rambam."

(There's another sage, Nachmanides, known as "Ramban." Most people stress the second syllable in "Ramban" and the first in "Rambam," to make them easier to distinguish.)