Here is what I do, I am always cautious about my choice of words.
I say a "Tiddlywiki instance" for any tiddlywiki, - file or folder based. Remember that it is irrelevant for most of us whether or not it is a file or folder based because it can be either or both.
If and when I need to differentiate, I use "a file [based] tiddlywiki " or "a folder [based] tiddlywiki"
However there are some cases where a "tiddlywiki file" is used simply because that is how we differentiate it, such as exporting a folder based wiki to a tiddlywiki file, or "send me a tiddlywiki file".
I would advise against simply saying a html file, because of my own initiative to also make the .tw extension a valid file extension, in part because html files are for other things as well, this is in part to support using tiddlywiki as smart documents (see other posts of mine). However a html file is an effective description of the technology standard used if it is .html or .tw. We may say "Tiddlywiki is a self contained wiki in a single html file".
I also use the word a "Served" [tiddly]wiki, which could be file or folder based tiddlywiki's, it is just that they are available at a http or https address be it a domain address, local host (rather than as a file:// address in a browser).
Regards
Tony