A Minimal Marketable Feature (MMF) is the smallest piece of functionality that can be delivered that has value to both the organization delivering it and the people using it. The Lean-Startup community sometimes call this a Minimal Viable Product (MVP) to underscore that one should come up with something as quickly as possible. The Agile community sometimes speaks of a Minimal Viable Feature (MVF) meaning what can be built to validate we're moving in the right direction. In any event, the MMF is a concept meaning - build the smallest thing possible to get value as soon as possible. This can be for release, discovery of what to build, or validation of the path you are taking.
Watch this 10 minute video - Minimal Marketable Features: The Why of Enterprise Agility - that discusses how to use MMFs to drive the return of business value quickly. It is important to note that epics are not MMFs - see More Insights on Epcis Vs Minimal Marketable Features.
For more on Minimal Marketable Features, you can read our short article The Business Case for Agility or the more in depth chapter from our book Lean-Agile Software Development on the Business Case for Agility.