ISO 9241-210 defines user experience as "a person's perceptions and responses that result from the use or anticipated use of a product, system or service". So, user experience is subjective and focuses on the use.
The additional notes for the ISO definition explain that user experience includes all the users' emotions, beliefs, preferences, perceptions, physical and psychological responses, behaviours and accomplishments that occur before, during and after use. The notes also list the three factors that influence user experience: system, user and the context of use.
Note 3 of the standard hints that usability addresses aspects of user experience, e.g. "usability criteria can be used to assess aspects of user experience". Unfortunately, the standard does not go further in clarifying the relation between user experience and usability. Clearly, the two are overlapping concepts, with usability including pragmatic aspects (getting a task done) and user experience focusing on users’ feelings stemming both from pragmatic and hedonic aspects of the system.
In addition to the ISO standard, there exist several other definitions for user experience. Some of them have been studied by Law et al. (2009