What's the defining feature of an MMO game in general?

What's the defining feature of an MMO game in general?
How we can define the MMO genre in general? What is the core difference between MMO games and non-MMO games? My answer is...

"Hive" - a hypercasual prototype

"Hive" - a hypercasual prototype

Player is controlling the "Hive" in the center, it's constantly gaining energy and can grow when fully charged.

Enemies are attacking the Hive and player have to spend the energy to defend itself.