Definition

A taxis is an innate behavioural response to a directional stimulus (a stimulus from a particular direction) for which the organism either moves toward (positive taxis) or away from (negative taxis) the stimulus, called orientation movement . For example, organisms in the genus of flagellate protozoa Euglena moves towards a light source. Here the directional stimulus is light, and the orientation movement is toward it. This is positive taxis to light, more specifically positive phototaxis .

There are four different forms of taxis:

• chemotaxis
• mechanotaxis
• haptotaxis
• durotaxis

Reference

http://www.chemistrydaily.com/chemistry/Taxis