Veronica dives into a swimming pool on a chilly winter day. initially, she feels very cold and begins to shiver. however, after a while, veronica stops shivering as her body begins to use energy to maintain its normal temperature. this phenomenon can be best explained as:

Homeostasis

Homeostasis is the balancing, through regulation, of the internal body environment to maintain the stable conditions necessary for proper body functions. The shivering is a first response by Veronica's body to maintain body temperatures.
However, when another homeostatic process begins such as the constriction of blood vessels, the body can maintain  heat without shivering. Shivering is a high energy consumption process.