This is a few confusing article, the words are so interchangable it is almost impossible to choose. Realms, Kingdoms, Lands, Regions and Worlds have no distinct characteristics on the show and as it has been mentioned not everyone can agree what defines each.
As far as I am concerned, the terms define different areas depending on the "world" you are in. Lets take Fairy Tale Land. This is a planet similar to our Earth and has several continents on it. Enchanted Forest (Misthaven), Agrabah, Arendelle, Camelot and The Empire. Now Arendelle and Camelot are the names of the specific kingdoms in those countries (We have evidence on the show that this is most likely) We call the countries Arendelle and Camelot simply because this is the only thing we get told by the show. We know Arendelle has several other nearby kingdoms such as Hans, Weasleton etc and we have only ever seen a single kingdom in Camelot so we can assume there may be other smaller kingdoms in the area such as Prince Johns kingdom from Robin Hood etc, Dunbroch is probably still part of the country known as Camelot but is ruled by its own royal family and is divided by sea. (similar to the isle of man in the UK)
Lands is a term to describe all areas of a country or can be used to describe a specific kingdom, i.e the land of Arendelle or the land of Agrabah, or the Kingdom of Snow and David or that of Prince Thomas and Ella. Same as the world Realm, this can be used to describe a land or specific kingdom. These two words are the most general and can be used to describe most places in the Fairy Tale Land. I think Realm is a little more specific then Land in the sense realm describes a magical land, or area. It rarely is used to describe the LWM. Storybrook is prob the only Realm within LWM as it is a magical area created by the curse and is not officially part of the LWM.
Kingdoms are specific areas rules by specific people and usually are a few towns/villages with a larger city like area which houses a castle/palace. We have several kingdoms in each country, Mistaven: King Leopolds, Queen Evas, King George, King Midas, Prince Thomas, Prince Philips, King Stefans, Prince Erics, Princess Rapunzels King Xaviers, Sir Maurices and Sir Gastons. Agrabah: Princess Jasmines and Jafars. Arendelle: Arendelle, Prince Hans, Wealston, Southern Isles and Northern Isles and Camelot: Camelot, DunBroch, Prince Johns (maybe). How the Empire is split I have no idea lol.
The whole of the land is what we should call a world, as it can be split into seperate areas. Despite the name having the word Land in.
Places like LWC, Wonderland, Oz, LWM, LoUS, Wish Realm, Dream World and Dark Realm are all worlds despite having the word Land in them. These are specific places that can be split up into different countries and or kingdoms. I.e LWC is prob similar to our earth, Wonderland is split into different kingdoms run by Queen of Hearts, Red Queen, White King and there is prob kingdoms for Spades, Diamonds and Clubs. Oz is split into North, East, South and West with different rulers for each etc.
Places like Neverland, Mount Olympus, Underworld, Netherworld, Worse Place, Kansas, Victorian England, 1920's England, 19th Century France, World behind the Mirror and Kansas are Lands. These are rules by a single kingdom or ruled by a single person or entities. I.e The Shadoow, Zeus, Hades, Queen Victoria etc. These are areas that are not split up into seperate countries or kingdoms.
I think I may have confused a few people and if I did I am sorry. These is how I define the lands and the worlds and the kingdoms.
Oh and a point I would like to make is that people seem confused about how David was given the role of knight of the Enchanted Forest and not the title of a knight from his own kingdom. Arthur does mention that a few of his knights are kings and princes of other kingdoms, though he does not mention which kingdoms. Knights from the Enchanted Forest would make sense and it seems logical that the knights he is referring to would hail from the forest as it is very close to Camelot. but it seems unlikely given the title he gives David. We know he has only 8 seats is it? Or close to 8 (I can not remember) One seat for himself, one for Lancelot, one for Percivil and one for Kay, so only four seats left, we also don't know how many are for these kings or princes and how many of the seats left belong to other Camelot knights. Does this mean the knights he is referring to hail from other kingdoms in the other countries, from Arendelle or from Agrabah? Possibly but again seems unlikely given the distances of these other countries. Could he be referring to kings and princes of other kingdoms in Camelot? such as Prince John or Sir Hiss (Or whatever his real name would be as chances are he would not be a snake lol) this to me makes the most sense. Am I making sense? I hope I am lol. If not I blame it on being tired. :)