There are some good “internet encyclopedias” for various topics, for example mathematics has ncatlab, philosophy has the Stanford Encyclopedia of Philosophy and literature has the Literary Encyclopedia. These are really good sites because they are free or near free and they use a really scaled back simple text format so they load quickly and are easily accessible everywhere. Is there anything similar for music? I’m not talking about Oxford Music Online, it requires a special account to log in and it’s slow to load in my opinion because the site is much more content rich.
Outside of Oxford Music, I'm not aware of any similar dedicated music encyclopedias. Wikipedia is as close as I know of, and it is generally comprehensive if not as authoritative as OM.
The only other encyclopedia-like sites I've found are:
- https://www.dolmetsch.com/index.htm: This site appears to be for a commercial early instrument seller, but they have a reasonably impressive "encyclopedia" (my word) built in.
- https://musicbrainz.org/: Focused around artists and recordings, an initial peek makes it look quite extensive in that regard, but you can't look up general terms. So, for example, I found Karlheinz Stockhausen (as well are various other musicians named Stockhausen), but there's no way to search for a term like "figured bass". And they seem to pull their encyclopedia-type information (e.g., artist biographies) from Wikipedia.
Everything else is restricted access (e.g., Music Online: The Garland Encyclopedia of World Music, a repository or research articles which seems not to allow for individual access), or not an encyclopedia. There are plenty of dictionaries, glossaries, bibliographies, indexes, and catalogs, but not the kind of encyclopedia you're looking for.
EDIT: Although not an encyclopedia, Open Music Theory could be put to similar purpose. It's an online, open-source textbook. It meets the "free and fast" criteria, but lacks a search function. One must rely on the table of contents to find things.
EDIT: Here's an online encyclopedia for microtonal music theory.
I recently discovered musipedia which describes itself as The Open Music Encyclopedia.
I don't yet know how comprehensive it is but I have found it to be a very good look up tool at least. You don't get a lot of information but you get some links which may be useful.
I have tried it mostly with classical music and, so far, I have not defeated it. I need to try some more obscure pieces.
I found it thanks to this earlier question.
It's not a specialized music encyclopedia, but Wikipedia covers a very wide range of musical topics.
Music-Map describes itself as the "Tourist Map of Music", I am not sure how comprehensive it is, but it's certainly interesting.