thermotropism

/ˈθʌrməˌtrɑˈpɪzəm/

When a plant's leaves curl in response to a chilly window, it's an example of thermotropism, which is an organism's tendency to move toward warmth and away from cold.

In biology, a tropism is an involuntary response to a stimulus. Once you know that, you can probably easily guess that thermotropism is a tendency to respond to (and usually grow toward) heat, since thermo means "heat," from the Greek thermos. It's mostly a habit of plants, though it might also explain the popularity of saunas on very cold days.

Definitions of thermotropism

1. noun

   an orienting response to warmth
   type of:

   tropism

   an involuntary orienting response; positive or negative reaction to a stimulus source