Ok, I know an island is land surrounded by water.
But if a river is separeted in two channels and then brought back together is the land in the middle an island?
What about a castle surrounded by a moat?
Well, if ya wanna get technical… they both are islands… and island is a general term… an island can be anything surrounded by anything else for example, a pile of sand in a meadow can be “an island of sand”
Being a man, I know that I am not an island.
Unless you live on the Island of Man.
That’s pretty much the situation at the city of Montreal, which sits on Montreal Island.
Ed
Is there any place which is at times, definitely an island, and at times, definitely not an island, because of the tides? I mean, a place big enough to have a name, like Sometimes Island.
Come on, folks, we can do better than this. All the land on earth is surrounded by water. That isn’t good enough.
Jim
Actually, the standard definition of island is such that it must be surrounded by water at low tide.
The UN Law of Sea Convention (which can be viewed here) defines an island that way, anyhoo.
The “traditional surveyor’s definition” (see here) states that an island must be separate and distinct from the mainland, and land must extend beyond the high water mark.
And now what are we going to do with this information? 
I know of a place named Wedge Island, which is just off the coast of Western Australia, that is mostly always an island, except for every 7 years or so. Then it becomes part of the mainland. I have absolutely no idea why that happens.
This is the definition Merriam-Webster had for an island:
It’s not very helpful, I know.
Presumably there is some lower threshold of size. Although I do not know what that might be. I’ve been to this island called Flat island. Which sits basically two feet above water and is about 30 feet by 30 feet in size. Perhaps a good rule for islands of small size is it’s an island if anyone bothers to name it. Otherwise it would be just a rock.
Nature is not so easily pegged. And attempts to define must surely end in tragedy.
[This site](Largest island in a lake on an island in a lake on an island - Topografisch Verbond ElbruzTopografisch Verbond Elbruz and Lakes.htm) from this thread
shows the Largest lake on an island,
the Largest island in a lake,
the Largest island in a lake on an island,
the Largest lake on an island in a lake,
the Largest lake on an island in a lake on an island,
the Largest island in a lake on an island in a lake,
and the Largest island in a lake on an island in a lake on an island.
Try to find a definition for that! 
One would have to add to any definition “…and smaller than a continent” to make the distinction. (As for what a continent is, and how it differs from an island, one million square kilometers is a handy cutoff, or one can use the geomorphological truism that every continent includes at least two distinct cordilleras, while no island does unless the highlands at the eastern and western edges of Greenland are so considered.
Most obvious example of such a place, now permanently connected to the mainland by causeway but otherwise known to have fit the “tidal island” criteria since the 1200s, is Mont St. Michel in Normandy, France.
How about this one for an islan? Most of the Eastern United States and all of Canada south of the Saint Lawrence Seaway. There is a string of water that surrounds the entire area starting at the ocean, going north the Mississippi to the Illinois to the Chicago River which gets to Lake Michigan and then through the Saint Lawrence Seaway back to the ocean. Is that an Island? It certainly is no continent.
It’s a bank
I believe an island must have vegetation on it, otherwise it is called a “dry ledge” or something like that. Anyone from the coast of Maine here? There are tens of thousands of islands and dry ledges (meaning small out-croppings or rocks surrounded by water at all times, even high tides) along that coast, and I believe to be called an island on a navigational chart the land must have some sort of permanent vegetation.
No man is an island but some women are incontinent. 
**Geologist Jomo sensed there was something not quite right with this theory, and went looking to find exactly where the fallacy was.
There is this concept in topography, you see, called a “divide.” It’s a line traced along relatively high ground: streams on one side of the divide flow down one watershed, streams on the other side flow down a different watershed.
In the case of the Illinois River, its headwaters rise on the side of the divide that flows into the Mississippi River and the Gulf of Mexico. The Great Lakes and the St. Lawrence Seaway are on the other side, flowing to the Gulf of St. Lawrence. The divide is only a few miles from the shore of Lake Michigan, at Summit which borders on the city limits of Chicago. The divide is low here, only 15 feet higher than the lake level. The Ship and Sanitary Canal from the lake, along the Des Plaines River to Joliet where it joins the river, is what connects the St. Lawrence and Mississippi drainage systems.
So the question is: does the Chicago Ship and Sanitary Canal turn the eastern United States and southeastern Canada into an “island”? Does a manmade canal really count?
It depends on whether people want it to count. Usually not, since the bit of land was not considered an Island before the canal was dug.
Sometimes it goes in the opposite direction. Coney Island, for instance, isn’t a separate island now, since the strait that separated it from Long Island has been filled in.