Linux users can try "dict", which is great, although weirdly it queries a network server for this information.

Well, the client and server are distinct. For a desktop system you can simply install both dict (client) and dictd (server), along with the various dictionaries.

The C/S element comes in handy on systems where the client is available, but dictionaries aren't. So from MacOS I can query my local Linux box.

(Debian has numerous dictionaries, including the US Census Gazeteer files, and others --- any information that can be represented in the dict format and be keyword-queried, basically. Quite handy.)

Huh, interesting. I imagine a lot of knowledge bases–personal, corporate, etc.–might be represented in that format.

Quite.

A principle limitation is that there's no security component, so anyone with segmented knowledge will not be having a good time. But pretty much anything else.

Using dict you can specify the target dictionary using the '-d <dictionary>' option. This can of course also be aliased or written into shell functions.