MMA are commonly used to denote a large, metropolitan municipal entity.
Is there a difference between the two? Is there a certified definition to these terms? Google is a bit inconclusive about the issue.
I'm assuming you're referring to specific to the USA. After much searching I came across this page: http://www.census.gov/population/www/documentation/twps0006/table03.html which states:
I can't find anything official for DMA. It seems to be used unofficially for "District Metropolitan Area", specifically Washington DC usually but that's it.
Found the same definition for MMA as GIS-Jonathan. For DMA though, I found this at Wikipedia.
Could be the MMA you're referring to.
In a Marketing context, DMA are Designated Market Areas.
The actual limits are updated every Fall by Nielsen, and there were about 210 of them last time I checked.
Here's one simple raster map of DMAs.