Final answer:
Taxis is defined as movement with a specific direction towards or away from a stimulus. It can be in response to light, chemical signals, or gravity. An example is positive chemotaxis exhibited by Tetrahymena thermophila.
Step-by-step explanation:
The subject of this question is Biology.
Movement that has a specific direction, either towards or away from a stimulus is called taxis. This movement can be in response to different stimuli such as light (phototaxis), chemical signals (chemotaxis), or gravity (geotaxis). It can be directed towards the stimulus (positive taxis) or away from the stimulus (negative taxis).
For example, a unicellular organism called Tetrahymena thermophila exhibits positive chemotaxis by swimming using its cilia. It moves in a straight line or makes turns depending on the stimulus. The organism follows the increasing concentration gradient of a chemotactic agent towards the source.