Maariv – the evening prayer service
Macher -- also Big Macher: Big Shot. Literally, a maker or doer. An important person.
Machmir – strict or stringent
Maikel – lax or permissive
Makpid – exacting when it comes to details, rules, regulations
Mamash – Yeshivish, means “REALLY” i.e. I mamash liked him
Mashiach – the Messiah
Mashpiim – a Mentor, or Rabbi, or someone to turn to for advice
Masmid – someone who learns full time
Mazel Tov- literally "good luck", or congratulations
Megilah - literally "story" or "telling"; usually refers to the story of Queen Esther and Mordechai
Mekabel – literally means to receive, but means also to accept something as fact
Menachem Avel - comforting a mourner, an important mitzvah
Mentch- a Man. Someone to be proud of.
Meshuga- Crazy. Also: Meshugas (me-shu-gahss)- craziness, insanity, and Meshuganer- a crazy person
Meshulach- an emissary, one sent to collect funds
Mevatel – self-nullification, lit. "wasting"
Mezuman- the 3 people needed to say a special blessing before saying grace after meals
Middos – personality traits
Mikvah- ritual bath
Mincha- afternoon prayer service
Minhag- custom, pl. minhagim
Minhag hagoyim - a non-Jewish custom
Minyan- literally, a quorum. The number of men (10) required for a formal prayer group. Also the name of the group itself.
Mir – a famous litvish yeshiva, now based in Israel
Mitzvah- Commandment (from G-d). pl. Mitzvot, Mitzvos
Mizbayach – the table in the holy ark in the temple
MO – Modern Orthodox, a stream of orthodoxy in which followers are equally involved in the secular and religious worlds
Morah - teacher
Moshiach Tzidkenu – The righteous messiah
Motsi Shabbos/Shabbat – Literally means "After Sabbath", refers to Saturday night after the Sabbath ends.
Mussaf- the extra prayer service after Shacharis, on Sabbath and Festivals