mm1 
abbreviation
Millimetre(s).
mm2
/m/(also mmm)
exclamation
1Used to express contentment or pleasure: mmm, that pizza looks so good mm, I love the sun...
- Mm, I feel much better now.
- Mm, I love french fries.
- "Vanilla," she murmured, taking one to taste. "Mm, delicious."
1.1Used to express agreement or approval: mm, yeah, I know what you mean...
- Mm, yes, I agree the review was a little basic.
- 'I brought my iPod, do you want to have a listen?' 'Mmm, okay.'
- “Mmm, that's right,” agreed Peter.
2Used to express uncertainty or reflection: mmm, I haven’t read the whole article but I don’t really like the sound of this...
- Mm, how does that work?
- Mm, I felt like I was watching a different show to the one you're all talking about.
- 'I could really see him as James Bond.' 'Mmm, not sure.'
MM3
/ɛmˈɛm/abbreviation
1Maelzel’s metronome (an indication of tempo in music, given as the number of beats per minute).
With reference to the metronome invented by Johann N. Maelzel (died 1838)
2Messieurs.
3(In the UK) Military Medal.