The Collaborative International Dictionary of English v.0.48:

Phototaxis \Pho`to*tax"is\, Phototaxy \Pho"to*tax`y\, n. [NL.
   phototaxis; photo- + Gr. ? an arranging.] (Biol.)
   The influence of light on the movements of low organisms, as
   various infusorians, the zoospores of certain alg[ae], etc.;
   also, the tendency to follow definite directions of motion or
   assume definite positions under such influence. If the
   migration is toward the source of light, it is termed

   positive phototaxis; if away from the light,

   negative phototaxis. -- Pho`to*tac"tic, a. --
      Pho`to*tac"tic*al*ly, adv.
      [Webster 1913 Suppl.]