What is defined as a regular response to a repeated, specific stimulus?

The correct answer is habit. A habit is a behavior that is regularly performed in response to a specific stimulus and often occurs automatically or unconsciously due to its repetitive nature. These responses are typically formed through consistent reinforcement and practice, making them engrained in an individual’s routine.

In differentiating from the other options, a pattern of behavior refers more broadly to the observable actions and reactions over time and does not necessarily imply a regular response to a specific stimulus. A routine typically involves a series of actions regularly followed, which may or may not include a habitual response to a particular stimulus. Lastly, a custom is a practice or tradition shared by a group and may not refer to individual behavior in response to a specific stimulus. Hence, "habit" aligns most closely with the idea of a regular response to a repeated and specific situation.