M vs thousand

M and thousand both are nouns.

Word	Noun	Adjective	Verb	Adverb
M	Yes	No	No	No
thousand	Yes	Yes	No	No

As nouns, M and thousand are synonyms defined as:

Other synonyms of M include 1000, chiliad, G, grand, K, one thousand, thou, yard.

M (noun)	thousand (noun)
concentration measured by the number of moles of solute per liter of solution	the cardinal number that is the product of 10 and 100
the 13th letter of the Roman alphabet
the cardinal number that is the product of 10 and 100
a unit of information equal to 1024 kibibytes or 2^20 (1,048,576) bytes
a unit of information equal to 1000 kilobytes or 10^6 (1,000,000) bytes

M (adjective)	thousand (adjective)
	denoting a quantity consisting of 1,000 items or units