How often have you struggled to explain what you do to a family member, a friend in another country, the hobo living under the bridge?
This is a definition I pickd up from the comments of this entry: http://www.webword.com/wp/2008/07/16/what-is-usability/#comments
"Users need to handle two basic problems:What should I do? Which is defined by the user (or for the user by someone else) and how do I do it? Which is defined by the application creator.Better usability results in less allocation of mental resources to answer this question.So, basically, usability is about reducing the users’ mental burden by providing good answers to the ‘how’ problem, thus allowing them to concentrate on the ‘what’ problem."
Do you agree? How would you define it in 5 or less sentences?