Major

1. one of four annual tournaments (The Masters, U.S. Open, British Open or PGA)  2. could also refer to a tournament of greater significance than usual due to tradition or the sanctioning body

Makeable

(fairly subjective) commonly used by commentators in broadcasts, refers to a putt (or short game shot) that is relatively straight forward and predictable in terms of the read (e.g., slope, speed) but with a length which would not necessarily suggest good odds on holing

Mark

to put down a ballmarker (usually a small flat object, like a dime) to be able to replace the ball precisely in its original location after lifting (usually, but not exclusively, on the green) (also "spot")
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Marker

usually a small, flat object (like a dime) used to mark the ball's position (usually, but not exclusively, on the green) while other players putt and/or the ball is cleaned (also "ball marker")

Markers

two objects that indicate the foreward boundary of the teeing area (also "tee markers, blocks, tee blocks")
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Marshal

1. a person designated by a tournament committee to help with crowd control and to keep order  2. a person designated to patrol the course, keeping an eye out for problems in general, but usually present to promote a reasonable pace of play or keep things moving

Mashie

archaic term for a 5 iron

Mashie iron

archaic term for a 4 iron

Mashie niblick

archaic term for a 7 iron

Match play

scoring by holes rather than strokes, the way golf was originally played

Medalist

the player in a stroke play, or medal play, competition with the fewest strokes/lowest score, the winner

Medal play

scoring by the total number of strokes (also "stroke play")

Member bounce

a very favorable (lucky) bounce or kick of the ball (also "member's bounce, lucky kick")

Mid mashie

archaic term for a 3 iron

middle iron

generally irons 5 through 7, which are in the middle of the range of irons in terms of length, loft, etc. (also "mid iron, mid-iron")

Misclub

choosing the wrong club (usually associated with good execution but an incorrect distance result)

Mishit

to not hit a shot solidly

Misread

to incorrectly assess the break of a putt or chip shot

More club

using a longer, less lofted club (also "club up")

Moving day

Saturday of a tournament taking place Thursday through Sunday, refers to the fact that it is common for a field of professional golfers to go through a substantial reshuffling in the standings once the cut has been made and as the final day of a tournament nears

Mulligan

taking a second attempt (replay, "do over") at a shot when one doesn't like the result of the first attempt (also "breakfast ball, lunch ball, Sunday ball")

Muni

short for a municipal or city owned golf course

Muscle back

a style of iron head with a thick amount of metal on the lower half or two-thirds of its back side (thicker than a traditional blade iron and almost the opposite of "cavity-back" or "perimeter-weighted" designs) (also muscle-back, muscle-backed")

Muscle memory

a phrase referring to the nervous system's ability to memorize, or perform automatically, a well rehearsed motion