Different areas have different conventions, leading to some confusion. Finance uses MM for a million, the physical sciences use M. In the the finance case 'M' comes from the Latin 'mille', meaning thousand, so MM means thousand thousand. In the sciences M means Mega (from the Greek meaning 'Great'), which is the SI prefix for 10^6

dagw answered this. "MM" is one convention for million, and somehow over the years I've adopted it.