Mystery meat – A term given for an option that you don’t know what it’ll do until you choose/select/click it. The metaphor as I underswtand it: You’re choosing tinned meat, but don’t know what’s inside until you buy it and open it.
I especially hate this used in navigation, where in recent years it’s become common practice to replace words with icons, to easily translate the interface into different languages, and fit more onto the screen for mobile.
Hidden salad – I went to a resetaurant recently and asked for a side salad with my burger instead of fries. The burger wasn’t supposed to come with a salad, so I expected to pay extra. As it turns out, the salad WAS a legitimate option, but a hidden one.
“Hidden salad” is what I call options that are valid but completely hidden from the user. Similar to mystery meat but instead of being presented with a cryptic option, you’re not given any option at all until you discover it. This is used especially in interfaces where you have to type, like command-line tools (which I especially hate). Or a more fun use might be Google’s hidden easter eggs.