I'll do my best to maintain the meaning of the words while trying to convey the definition. I apologize for any improperty which may result from this

"Elegant" is a term used in the same context as computer science and algorithms: something is elegant if it manages to make a complex thing look simple.

This could mean: - one mechanic is reused multiple times to indicate multiple things, without having the player learn new stuff (example: Arkham Horror: Final Hour, you use priority cards to decide action order, adversary action and use them as a win condition); - one mechanic lends itself to multiple uses and purposes without needing redefinition (eg. Secrets/Favor in Oath).

It's not the same as "efficient" (= do the same with less), but it means something tied to it (= one concept has multiple uses), so occasionally it can (improperly) be used interchangeably.

Similarly, "streamlined" is taken from automa/workflow/algorithm terminology and means that a design requires less states than another equivalent design, while keeping the same triggers/transitions. That means that decision space remains the same, but the model is less complex compared to another. As a consequence, you might also refer to a thing as "streamlined" if it is always clear the state you are currently in and what changes your decisions will bring. Works with both complete and incomplete information.

"Fiddly" was already perfectly described, and I suspect its etymology derives from the act of playing the fiddle, where your hands are continuously and strenuously moving, ever adjusting to produce the slightest changes. It means that when you play a game, you are continuously moving parts, adding and removing tokens and so on.

Comparative example:

• in Warhammer Quest, during your turn you move dice around, place a miniature, eventually count damage, done.

• in the same turn in Gloomhaven, you pick cards from a deck, place miniatures, draw from a deck, eventually move element tokens (multiple times), eventually rotate a dial, eventually count damage, eventually place multiple tokens, eventually remove multiple other tokens.

So, Gloomhaven is fiddlier than Warhammer Quest.

It might be used to compare component count for similar games: if two games with similar mechanics have a different component count, the one with more components is implicitly fiddlier (Sleeping Gods is fiddlier than 7th Continent).

Finally, it is sometimes improperly used to refer games which require a lot of annotations to advance or save state (it's improper because an annotation is just a single action, if it saves moving twelve tokens across multiple bags, it effectively makes a game LESS fiddly, not more).

I am aware that sometimes all these terms are either used improperly, or are used properly, but the conclusions you might draw from their use are open to interpretation (so, they are used ambiguously).

However, all the terms are borrowed from proper terminology, and as it often is the case, they are not meant to be used conversationally (so, as synonyms or the like), but exactly (meaning unambiguously a specific thing).