Both microorganisms and higher organisms display a variety of motile responses to light as an environmental signal. Such light-elicited motility responses are generally referred to as photomovement. The organisms use the photomovement to adapt to optimal physiological and growth conditions. Phototaxis refers to the movement of a motile organism toward (positive phototaxis) or away from (negative phototaxis) the source of light. Rigorously speaking, the positive and negative phototaxis refers to the organism's response with respect to the direction of light source. However, the term phototaxis often is used in reference to the light-elicited movement behavior of a population of organisms toward or away from the source of light, which may or may not be in response to the direction of light propagation. Phototaxis can result from different mechanisms of the light-elicited movement.

For example, an individual ciliate cell may stop its swimming upon encountering a sudden increase in light intensity, and it then steers away from the higher to the lower light intensity area. This response is called step-up photophobic response, and results in a negative phototaxis, as the cell swims away from the high-intensity light. To define various light-elicited movements of organisms that may result in phototaxis, it is necessary to discuss various photomovement responses. The photomovements of specific ciliates and bacteria will be used as typical examples.