Time to embark on a not-very-ambitious project: I'm going to try to define, explain and justify the merits of some core concepts I wish everybody knew. The ideas that people have come up with a word for, and having that word lets you manipulate that idea in your head more easily.
I am going to start with the conceptual model. I think I first encountered the term in Donald Norman's The Design of Everyday Things, and while I thought it was useful at the time, I've come to find it an indispensable idea.
The Design of Everyday Things is about interaction design. Its core thesis goes something like this: human beings have a lot of physiological and psychological similarities, and we can use those similarities when designing objects for human use. The very shape of a well-designed artefact should tell its user how to use it.
An everyday example is doors. Do you push or pull on a door to open it? Norman firmly insists that you should never need "PUSH" or "PULL" signs on a door. If the door has a plate, the only option you have is to push it. If the door has a handle, it is affording you the option to pull it. A door built this way tells you how to use it1. Its shape gives you a conceptual model of how the door operates: you've got a pushable thing on this door, and if you push it, the door will open. If it has pull handles on both sides but only opens in one direction, it would be giving you a bad conceptual model that will lead you to misuse it.
Doors are quite a simple artefact. A more sophisticated example would be a thermostat. The typical thermostat works to known principles: you set it to a temperature, and heating elements will turn on or off to regulate the room to that temperature. There generally isn't any gradation in the heating elements; they're either on or off. In spite of this, thermostat users still commonly turn the thermostat up to a higher-than-desired temperature in the belief that it will heat the room faster.
What's gone wrong here is that the user has a bad conceptual model. They think the temperature of the thermostat is the temperature of the heating element. If this were the case, turning the thermostat to a higher temperature would indeed heat the room faster, but it's not the case, so they get undesired and confusing results from their thermostat.
Conceptual models play an important part in design. Today millions of people carry around incredibly sophisticated computing devices in their pockets, but the principles of interaction design make their use feel intuitive. Almost nobody using Twitter has any idea of how it works, but it's built with a very straightforward conceptual model that people can grasp without difficulty.
It's worth mentioning that conceptual models don't have to accurately reflect what's going on "under the hood" in order to be useful. Your computer, for example, doesn't store its data in any way that resembles actual files and folders, but the intermediary file system provides a conceptual model that allows you to abstract away all the ones and zeroes.
Design is not the only reason for thinking in terms of conceptual models. In the broadest sense, any model of how a concept works is a conceptual model2, but the design context is a very apposite one. When a user has a bad conceptual model of how a piece of technology works, they struggle to use it. They bang their head against it and call it all manner of nasty names. In my experience, this is also what people do when they have a bad conceptual model of more abstract concepts, like currency or calculus.
We're now getting to my motivation for wanting "conceptual model" as a thing inside everyone's head. I would love to be able to be able to ask "can you give me a conceptual model for how this works?" and expect a functional but not necessarily factually-correct story or metaphor that equips me to use it. I'd love to be able to say "I'm giving you a conceptual model for this", and have the person I'm talking to realise that I'm giving them a tinker-toy explanation that works for most practical purposes, but shouldn't be thought of as "true".
Also, bad conceptual models are everywhere, and I would like to be able to identify them as such. When someone's labouring under a false assumption, you can say "you're labouring under a false assumption" and they will probably understand what you mean. I'd like that level of conciseness whenever someone (especially myself) is labouring under a bad conceptual model.
Conceptual models are a highly useful concept: download this app to your necktop.