Quasimode

Jef Raskin coined the word in his book The Humane Interface, from the Latin quasi, meaning almost: a quasimode is almost a mode, and the difference is who is holding it open. An ordinary mode is a state the software keeps. A quasimode is a state the reader keeps, with a finger, and it collapses the instant that finger lifts. Shift is the one everybody uses without noticing. Hold it and the whole keyboard means something else; let go and it does not.

The reason this matters is that modes fail by being forgotten. A mode error is what happens when a person acts on a screen that means something other than what they think it means, and almost every famous one comes from an interface that was quietly still in a state its user had stopped thinking about. A quasimode cannot be forgotten, because forgetting it means letting go of it, and letting go of it ends it. The cost of remembering has been moved from memory, where it is unreliable, into the hand, where it is free. That is the entire argument, and it is why holding space to pan a canvas, holding Alt to sample a colour, and holding a key to talk on a radio all feel safe in a way that clicking into a pan tool does not.

It is worth separating from its neighbours precisely. A chorded shortcut also involves keys held down, but it fires a command once at the moment the keys come together, and the holding is only how the keys are collected. A quasimode fires nothing: it changes what other input means, for exactly as long as the hold lasts. Jiggle mode is the counter-example on the other side, a mode you enter with a long press and then have to leave deliberately, which is why people get stuck in it. And a modifier key is the smallest quasimode there is, which is the neatest evidence that the pattern is already everywhere.

The honest caveat is physical. Holding a key while doing something else with the other hand is a two-handed gesture, and it is exactly the thing sticky keys exist to undo, so a quasimode must never be the only route to a capability. It also needs to say it is on, since a reader who has just discovered the mode has no way of knowing where its boundary is until something tells them, which is why the good implementations put a visible badge on screen for the duration and take it away at the release.