Attention, memory and perception
Discoverability
Also called Findability
How easily people come across a feature they didn't know existed, as opposed to findability, which is locating something they know is there.
A feature nobody discovers might as well not exist. Discoverability depends on placing features where people's tasks take them and signalling what's possible.
Introduce features in context, at the moment they'd help, rather than in a tour on day one that nobody remembers.
In practice
A spreadsheet that suggests a pivot table after someone sorts and totals the same column twice.
Common mistake
Hiding everything behind gestures or long-press menus to keep the screen clean, then wondering why nobody uses those features.
Where it comes from
Don Norman named discoverability as one of the two most important qualities of good design in The Design of Everyday Things. The distinction from findability is from Nielsen Norman Group.