The word Majority in Greek is Pleiropsifia
We think you mean πλειοψηφία,
pleiopsifia, which is in fact the modern Greek word for "majority."
Curious that the ancients had no single word for "majority rule" (which we assume you're trying to get at) as such. It may be that, no matter what the system of government, majority rule was a given for any ruling party, and didn't require a special name. Their expression for "the majority" was simply οἱ πολλοί,
hoi polloi, or οἱ πλείονες,
hoi pleiones.
Yes, you might try concocting a noun based on
archē and
pleiones, i.e.
pleionarchia, or, anglicized, "pleionarchy".