I. mm 1 BrE AmE
the written abbreviation of millimetre or millimetres
II. mm 2 /m/ BrE AmE
used when someone else is speaking and you want to show that you are listening, that you agree with them, or that you are thinking about what they have said:
‘That’s okay, isn’t it?’ ‘Mm yeah.’
Mm, I see what you mean.