The omniscient narrator gets its name because The Narrator knows and sees everything. (Other writing advice folk might disagree with saying The Narrator but let’s keep it simple.) The narrator in an omniscient narration is different from a limited omniscient narration. In the limited version the narrator can only see what is happening from the perspective of one character at a time and is limited in that moment to what that character knows. The omniscient part of limited omniscient comes from the fact that they can move between several characters thereby seeing more of the story.
One way to look at it might be as different lenses. The first lens is a microscope that focuses in on one little part of the story from the perspective of one character (first-person/subjective). The second lens is a magnifying glass; you’re still close but you can see more of the overall story (limited-third/limited omniscient). The final lens is a glasses lens, you can see the whole world of your story (third-person/omniscient).
There are so many varying degrees between limited omniscient and omniscient it can sometimes be difficult to judge where you are so I tend to work on the principle that if the narrator is a voice in their own right its omniscient. By this I mean that when we write limited omniscient we can keep the narration close to the character’s voice if we choose, this isn’t necessary, but we’re always limited to that character’s knowledge in that moment. In omniscient the narration has a voice of its own, which is the same for any character it focuses on and doesn’t change. Omniscient narration can also pause and say something like ‘Jane knew this but what she didn’t know was this’ or ‘Jane was thinking this, but John was thinking that’. Limited omniscient can only do this by repeating scenes in different chapters or sections to replay it from a different perspective or we’d be head hopping, which is confusing in limited omniscient and defeats the point of being limited.
Another thing an omniscient narrator can do that a limited omniscient narrator can’t is wander off completely from the main cast to tell another story without changing perspective. Terry Pratchett does this in the Discworld books where he may tell us a little of the mythology of the Discworld. In American Gods Neil Gaiman has chapters devoted to stories of how the gods came to be in America which, while they tell us more about the story, don’t always follow the gods themselves. If we were to apply the same principle to limited omniscient we’d either have to have chapters dedicated to characters who will only appear once, or the chapters would have to be from the perspective of the god it’s about. Both of these could be valid choices but would work differently from an omniscient narrator slipping back and forth through time and space. In a limited omniscient narration we might need a little time to adjust to a new perspective while in an omniscient narration the perspective wouldn’t change, unless the narrator chose to step into the character’s head.
Changing perspective in limited omniscient to a character who only appears in one chapter isn’t impossible in limited. However, it can be tricky to pull off because a reader can become confused by the shift or frustrated that the character they just got used to has disappeared and never reappears. This can also be problematic in omniscient but because we are further out and the perspective doesn’t change the effect can be lessened because it’s the same narrator carrying the story along so the break can be less conspicuous.
It’s important to note that while both types of narration have pros and cons they’re both valid. The distance we set our narrator from the story does not make one style better than the other it’s all a matter of preference and story. While a writer may prefer one style of narration most of the time it doesn’t mean they have to stick to it if they find a story that might work better with a different style. It can also be very beneficial to try all the styles of narration to find which we’re most comfortable with, even if we don’t like one we will still learn something new.