A "threshold" is a point or level at which something begins or changes. It can refer to physical boundaries, like the entrance of a room, where one crosses from outside to inside. In a broader sense, it can also describe limits in various contexts, such as the pain threshold, which indicates the minimum level of pain that a person can perceive. In scientific terms, a threshold can signify the minimum amount of stimulus needed to trigger a response. For example, in psychology, the threshold for detecting a change in sound volume is the point at which a person notices the difference. Understanding thresholds helps in various fields, including medicine, engineering, and environmental science.