There is also High Island, TX which is an ‘island’ of height in a ‘sea’ of flat land. A pedant might argue that it has been a real island since the Intracoastal Waterway was constructed, creating a manmade island of the entire coastline between East Galveston Bay and Sabine Pass. But it was just a high spot on the coast when it got its name.
See, I’m used to it. If the call Manhattan an island and even here in Chicago we have our Goose Island, then that’s an island, too.
No. That is clearly a deltoid landlet. Now watch how much success I have promoting the spread of this term.
So does that mean all of New England, Eastern New York and a piece of Canada is one big island? It’s bordered by the Hudson River, Lake Champlain, the St Lawrence River, and the Atlantic Ocean.
How would Manhattan not be an island? 
It’s not like the rivers surrounding it are shallow enough that they dry up and you can walk them or anything.
Personally, I would say that a canal with locks shouldn’t count for defining an island. One of the gates must always be closed which presents a solid connection across the canal. With that restriction, New England is not an island because Lake Champlain is connected to the Hudson River by the Champlain Canal.
Without that restriction, the entire eastern U.S. (plus some of Canada) is an island defined by the St. Lawrence River, the Great Lakes, the Chicago Sanitary and Ship Canal, the Des Plaines River, the Illinois River, and the Mississippi River.
Ha, great point. I propose that the minimum distance to the mainland in at least two directions be greater than the diameter of the island at its widest point.
What exactly is the diameter of Manhattan Island? Even if you mean the width at midtown, that’s wider than the Hudson. Of course if you let me take my two directions as SW and SSW, I guess it could qualify. How many degrees must separate the two directions?
Alameda in the San Francisco Bay Area is one of those. It was connected to the mainland until a channel was dredged for shipping. Oddly, Bay Farm Island, which is right next to Alameda, was once a true island but was connected to the mainland by landfill.
And yes, a piece of land surrounded by water is an island regardless of how it got that way.
Yet we say Australia is not an island. I don’t think the commonsensical understanding of “island” includes large swaths of land that happen to have rivers running in various directions.
Fine, let’s get exacting. 
First, we find the geographic center of the (candidate to be an) island. All subsequent measured line segments must contain this point.
The diameter is defined as the line segment which passes through the center and has the greatest span over land. “At least two directions” means two that are a minimum of 90 degrees from each other.
To me, it’s absolutely unequivocally an island. Is it that common to think it isn’t?
Do we?
Seems to me the definitions I’m advocating are no more arbitrary than these, and probably less. Also more commonsensical, since as you say it’s obvious to most people that Australia should be considered a very large island.
“No man is an island”
- Irving Peninsula
Sorry, but when I am treading water in a swimming pool, I am an island.
Incidentally, Boston was originally an island, but then landfill connected it to the mainland. Long Island is an Island, Manhattan is an island, Montreal Island is an island. Somehow, the canals that cut off the Niagara Peninsula don’t quite make it an island, not in my mind. What about Jutland together with that slice of Germany north of the ship canal? Nope.
For this argument, you chose the one continent literally called ‘the island continent’?
Are you sure? Everything I’ve seen of colonial-era Boston was that it was a peninsula attached by a very narrow isthmus called Boston Neck.
Rhode Island is separated from the mainland by a sea of ignorance.
Playing with “Measure Distance” on Google produces an area of roughly 3 square miles. That gives it a larger land area than Wake Island in the Pacific, Ellis Island in New York Harbor, and Alcatraz Island in San Francisco Bay, among others.
Technically (the very best kind of correct), the state is “Rhode Island and Providence Plantations”, of which Rhode Island (informally known as Aquidneck Island) is a part.