See also #usable.
Definitions:
Capability of a product to present functions and information in an inviting and motivating manner, supporting continued interaction. This refers to properties of the product that increase the pleasure and satisfaction of the user, such as harmonious colour, intuitive user interface or friendly voice guidance.
The name of “user engagement” replaces the previous name “user interface aesthetics” (in ISO-25010-2011) as quality subcharacteristic.
Note from ISO-25010:2023
Engagement is a quality of user experience characterized by attributes of challenge, positive affect, endurability, aesthetic and sensory appeal, attention, feedback, variety/novelty, interactivity, and perceived user control.
The ISO rename replaced the name, not the concept. Engagement is a behavioural outcome — users keep interacting, return, recommend — and user interface aesthetics is one of several properties that contribute to it. This site therefore keeps both as distinct qualities; the aesthetics page explains the difference in detail.