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I'm currently on holiday in Indonesia. In my travel guide I was reading about the island Samosir, which I'm visiting in a few days. I read that the island, which is located in lake Toba, is connected to the main island of Sumatra by an isthmus. This got me wondering whether this wouldn't technically make Samosir island a peninsula.
So my question is: what (if anything) is the technical difference between a peninsula and an island connected to its parent landmass by an isthmus?
I think the naming difference is largely qualitative and even arbitrary in some cases. There are lots of 'islands' that are technically connected to a larger landmass by an isthmus of some sort, which would make them peninsulas technically, and not islands.
If you take the definition of a peninsula to be a body of land surrounded by water on three sides, then yes, the island in question could technically be called a peninsula.
Local names of landforms are often linked to cultural or historic influences, just as much as they are to their technical description. There's no 'threshold' of isthmus-width (as far as I'm aware), where once crossed, a peninsula-like island becomes an island-like peninsula . If there was it would be entirely arbitrary.
As an interesting aside, the Isle of Harris in Scotland is not only not an island (there's an isthmus connecting part of it to a larger landmass) but even the isthmus part is not used to define the limit of the island, which occurs at some seemingly arbitrary point on the larger landmass! (see the map in the link to see what I mean)
In a topological sense, if it's connected by an isthmus then it isn't an island. That doesn't mean that it might not be called that - but the answer is a cultural & political one, and hence won't adhere to strict definitions.
