In reading product advertisements and gear reviews in magazines like Sound on Sound, I've come across the term "musical" a few times referring to an apparently positive quality of an equalizer device or plugin.
What does it mean for an EQ to be musical? Is there a generally-agreed-upon definition for this term, in this context? Is there such a thing as a deliberately "un-musical" EQ?
I think this is largely marketingspeak. And they stole it from engineers who were trying to convey that some EQ voicings are just more right than others when you're working with EQs that have limited controls.
In particular, when you've got an EQ that's not fully parametric, or configured with enough bands to cover the entire audible frequency spectrum, you're at the mercy of what some electrical engineer thought was the best parameters for Q and taper and whatnot on the EQ.
This is usually the case when you're working with channel strips on analog mixing consoles: each strip has an EQ section, but it's simple in what you can do because there's limited room on the physical strip for potentiometers to play with. As such, the channel strip designer has to make some tough decisions about how that EQ can be controlled and how each dial changes the EQ voicing. A "musical" EQ in this context would be one where, when you move a dial on the EQ, gives you pleasant results -- the Q and taper and center frequencies of the bands in the EQ all change in a way that makes working with the EQ easy.
Keep in mind that even EQs with fully customizable band parameters will not all sound the same when set to identical settings. The type of electrical circuit and quality of components (or the type of digital algorithm) used to achieve the equalization will be different for each EQ model, and these differences have very real effects on how the signal is processed. A lot of these show up as phase shifts or effects on harmonically-related frequencies that aren't actually in the intended band.
These differences are what make some EQs better than others for certain applications, and could be part of what the "musical EQ" is referring to.
Though the point Ian C. made is probably what is most often meant. I just posted this addition as something to keep in mind.
I hear that term "musical" a lot when it comes to effects. It's more of a "wow this makes me sound good and I'll be inspired by it" quality. I've used the term to describe effects as well. It's a feeling that you can really use the effect to a "musical" potential where it really adds something special to what you're trying to accomplish.