A pan that is wider than it is tall, usually with a long handle, and occasionally having an offsetting smaller handle (generally for heavier or larger pans).
Skillets are used when flat real estate is needed, but with a need to accommodate some liquids. Think of a skillet as a griddle with sides. That is why the skillet is in the essential category, but the griddle is not. You can make pancakes in a skillet, even though it would be easier with a griddle. You can't make chicken fried steak ? with a griddle.