thousand vs M

thousand and M both are nouns.

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

As nouns, thousand and M are synonyms defined as:

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

thousand (noun)	M (noun)
the cardinal number that is the product of 10 and 100	concentration measured by the number of moles of solute per liter of solution
	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

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