[wavefunction]

It can't be an abbreviation as that refers to subtractions of letters existing in the word and I only count one 'm' in millions.

Yes it's pedantry and yes I understand that 'mm' is in someways a convention but it's a terrible one.

[anamexis]

As long as we're being pedantic, abbreviation does not refer strictly to "subtractions of letters existing in the word," it is any shortened form of a word or phrase.

https://en.wikipedia.org/wiki/Abbreviation (see especially the section on plural forms)

http://www.oxforddictionaries.com/us/definition/american_eng...

http://www.merriam-webster.com/dictionary/abbreviation

[wavefunction]

How is adding non-existent letters any form of abbreviation?

That's all I was stating.

[anamexis]

Because it is a shorter form of the word it abbreviates. 'mm' is shorter than 'millions'.