[Etymology: notably from Lat: mille, = 1 000] The Roman numeral for 1 000.

(Metric) As an upper-case prefix, M-, see mega-, e.g. MHz = megahertz. Hence more generally as M, see million.

(Metric) As a lower-case prefix, m-, see milli-, e.g. mg = milligram.

North America As m, thousand, then mm = million and so on, particularly in business literature (and in contrast with the metric meaning of a thousandth), e.g. as in mFBM for 1 000 FBM of sawn wood, mm.c.f. for 1 000 000 cubic feet of natural gas.

length (Metric) As m, see metre. Also prefixed variously, as in mm = millimetre(s); see also SI alphabet. Strictly lower case.

UK On highway signs, as M, metre; as m, mile: an unfortunate practice when the lower-case letter is the only proper symbol for metre, but understandable given the widespread investment in and established use of m for mile.

electromagnetics (Metric) M is sometimes used without the alien Ω to represent MΩ, i.e. megohm.

electrics and mechanics [magnification] See Q factor.

astrophysics For stellar magnitude, M is absolute magnitude, m is apparent magnitude.

paper and printing Often used Roman-style for 1 000 sheets, etc.