the way a user interacts or reacts to an artifact and/or scenarios (i.e. combinations of stimuli + environment + circumstances) within a particular context.
Behavior is the range of actions and mannerisms made by organisms, systems, or artificial entities in conjunction with their environment, which includes the other systems or organisms around as well as the physical environment. It is the response of the system or organism to various stimuli or inputs, whether internal or external, conscious or subconscious, overt or covert, and voluntary or involuntary.
User behavior is behavior conducted by a user in an environment. In User Experience this could be on a web page, a desktop application or something in the physical world such as opening a door or driving a car.
Questions having the user-behavior tag regards users reaction and/or behavior to the environment she encounters.
Reference: Wikipedia - Behavior