M vs large integer

M and large integer both are nouns.

Word	Noun	Adjective	Verb	Adverb
M	Yes	No	No	No
large integer	Yes	No	No	No

As nouns, large integer is a hypernym of M; that is, large integer is a word with a broader meaning than M:

M (noun)	large integer (noun)
concentration measured by the number of moles of solute per liter of solution	an integer equal to or greater than ten
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