Mordan IV

M-113 – a Class M planet featured in TOS episode "The Man Trap". The planet is also called Crater's World, and also known as Fotialla by the extinct natives that once lived there.

M'kemas III – Tzenkethi settlement attacked by Ambassador Krajensky (a disguised changeling) who commandeered the USS Defiant in an attempt to start a war with the Federation.

M-S-1

M-Zed V – Mentioned in TNG episode "Heart of Glory". It is an outpost possibly under Klingon control. It was claimed as the destination of Korris and his fugitive Klingons. Note: It may simply be spelled as M-Z-V as "Zed" is the typical British/Canadian pronouncement of the letter "Z" instead of "zee".

Mab-Bu VI – Gas giant planet with a Class M moon. In the 19th century, criminals from the Ux-Mal system were exiled to the moon where their consciousness was separated from their physical bodies by the planet's electromagnetic fields. Some time in the 2160s, the prisoners attempted to escape by inhabiting the bodies of crew members of the Federation starship USS Essex but the plan failed when the ship crashed on to the surface. The beings later take possession of Data, Troi, and O'Brien to hijack the Enterprise-D.

Machine planet – So-called world where living machines built a gigantic vessel for the V'ger (Voyager 6) probe which came to Earth to search for its creator. Spock saw the machine-planet as a full-scale holographic image in V'ger's immense memory chamber along with other representations of objects that V'ger encountered along its journey.

Magna Roma – The fourth planet of the FGC-892 system, which is discovered with a human society modeled after the ancient Roman Empire of Earth.

Magus III – Guinan once broke up a fight in Ten Forward with a weapon she described as "a little souvenir I picked up on Magus III."

Makus III – Destination of the USS Enterprise NCC-1701 in TOS episode "The Galileo Seven". The primary city of New Paris, where they were supposed to drop off medical supplies before the Galileo shuttlecraft is lost.

Malaya IV – Planet with a Federation medical facility.

Malcor III – Third planet in the Mu Ceti system. Homeworld of the Malcorians who were on the verge of warp capability. Commander Riker goes to the planet disguised as one of the inhabitants but his cover is blown and causes a panic among officials when they discover an alien living among them.

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Wikipedia