Definition: A flat top is a type of cooking range whose surface is sort of a cross between a griddle and a grill. Unlike a grill, a flat top doesn't have a grate, but simply a flat cooking surface. And unlike a griddle, you can use pots and pans on a flat top, in addition to cooking the food directly on the surface.
Another difference between a flat top and a griddle is that a griddle is heated from below with straight heating elements that run the length of the griddle, whereas a flat top has multiple round heat elements.
It's basically an ordinary range top with individual burners and a continuous flat cooking surface over the burners.
This characteristic makes the flat top extremely versatile. For one thing, with an ordinary range, you can put pots and pans on the burners, but nowhere else. With a normal range, if you have six burners, you can fit six pans. But with a flat top, pots and pans can fit anywhere on the surface. Different regions on the flat top will be hotter or cooler, but it will accommodate many more pots and pans, in addition to being able to cook food directly on the surface.
With a flat top, the cooking surface itself is generally a heavier type of steel than found in a typical griddle, almost like cast iron. In fact, after cooking on the flat top for some time, it will start to become "seasoned" like a cast iron pan, giving it somewhat of a nonstick quality.