first moment vs mean value

first moment and mean value both are nouns.

Word	Noun	Adjective	Verb	Adverb
first moment	Yes	No	No	No
mean value	Yes	No	No	No

As nouns, mean value is a hypernym of first moment; that is, mean value is a word with a broader meaning than first moment:

first moment: the sum of the values of a random variable divided by the number of values
mean value: an average of n numbers computed by adding some function of the numbers and dividing by some function of n

Other hypernyms of first moment include mean.

first moment (noun)	mean value (noun)
the sum of the values of a random variable divided by the number of values	an average of n numbers computed by adding some function of the numbers and dividing by some function of n