I've mentioned Lord of the Flies as a perfect example of 3rd omniscient, not 3rd limited. If you read carefully, you'd notice that there's indeed an "invisible" narrator separate from all the characters. The narrator may choose to focus on Ralph, for example -- the narrator knows everything about him and what he's thinking or feeling and can also report on things Ralph doesn't know, but the narrator is not Ralph (or any other characters, for that matter), nor does it assume Ralph's personality or perspective. This narrative voice is established every early on and does not waver throughout the entire book.
In 3rd limited, such as Old Man and the Sea, the narrator is like a shape-shifter who assumes the identity of the character, but continue to refer the character in third person. The narrator is not, however, all-knowing. The narrator can only report on things the character knows and feels and thinks. Sometimes the narrator assumes the character's personality and voice -- for example, the simple prose of a fisherman. In essence, 3rd limited is kind of like first person -- you're limited to one character's perspective at a time. You can, however, switch between multiple POVs but only one at a time.
When you're switching at will (within a scene, etc.) then you're head-hopping. So how is 3rd limited with head-hopping different from omniscient? The difference lies in the discipline the authors has in maintaining a single, unified narrator. In that case, it's omniscient -- an all-knowing single narrator. We may not be able to tell the difference by reading an excerpt or two, but within the context of the entire novel, you'd know. The omniscient narrator doesn't waver, doesn't change voices or personalities based on the characters. The narrator is constant.
