Learnability is defined as the capability of a product to enable the user to learn how to use it. In UX design learnability is a key usability feature that is often disregarded.
The main reason learnability sometimes gets overlooked is due to the fact it’s relatively hard to test or quantify. In many cases, the only accurate way of testing whether a website or app is learnable is over a long period of time, using the same users testing the interface multiple times at regular intervals. This is the type of testing that sometimes gets neglected, as it can be time and money consuming.
It’s worth noting that learnability is not the most important feature for all interfaces and some applications will have a more extreme learning curve than others by their nature. For example, a simple e-commerce website should be primarily instinctive with possibly a few simple learnable extras, whereas a complicated application for invoicing and billing could require a high level of learning before the interface can be used at full efficiency. However, arguably all systems should use some principles of learnability in order to be as effective as possible.