I’ll start with a clarification: (just in case!) usability and user experience (UX) are not the same thing: the usability of a product is a crucial part that shapes its UX, and hence falls under the umbrella of UX.

While many might think that usability is solely about the “ease of use” of a product, it actually involves a great deal more than that.
The ISO 9421–11 standard on usability describes it as:

“The extent to which a product can be used by specified users to achieve specified goals, with effectiveness, efficiency and satisfaction in a specified context of use. ”

– ISO 9421 Ergonomics of Human System Interaction.

Utility + Usability = Usefulness
When you’re designing for usability, it’s important to think about utility, too. While usability is concerned with making functions easy and pleasant to use, the utility is about providing functions that users *need *in the first place.

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Only when usability is combined with utility do products become useful to their users. A mobile payment app could provide the most usable feature of adding the people around you on Facebook, but because most users of that app wouldn’t need that feature, it’s going to be useless to them.

All your effort towards building the most user-friendly feature could go to waste if that feature isn’t needed.

Read more on this topic here: What is Usability?