The theory that the power to control the Bitcoin protocol is held by those able and willing to offer things of value for bitcoins (be it goods, services or other currencies).
As long as mining is conducted for economic gain, then any change adopted by the miners needs to be supported by the economic majority for it to be successfully implemented.
If there are changes to the protocol that are not welcomed by those who want bitcoins but the changes are adopted by miners regardless, then the coins mined (starting with the first non-compliant block mined) would not be recognized by those who didn't adopt all the changes. Thus those miners would then, in that instance, not gain economically from introducing any changes that the Economic Majority didn't want. This outcome would be the same regardless of whether or not the miners that attempted to make the changes had 51% of the hashing capacity.
So the ability for a protocol change to be successfully implemented ultimately rests with those who accept bitcoins in exchange for value. Generally those will be the merchants. If the economic majority doesn't run full nodes Bitcoin is dead.
Credit goes to Meni Rosenfield for first coining the term.
- Is Bitcoin's Economic Majority those who already own coins or those who will buy or keep coins? on Bitcoin Stack Exchange