Merriam-Webster Collegiate Dictionary - taxis

 noun  (plural taxes)  Etymology: Greek, literally, arrangement, order, from tassein to arrange  Date: 1899  1. reflex translational or orientational movement by a freely motile and usually simple organism in relation to a source of stimulation (as a light or a temperature or chemical gradient)  2. a reflex reaction involving a ~ TAXIS  noun combining form  (plural -taxes)  Etymology: New Latin, from Greek, from taxis  1. arrangement ; ordering thermotaxis  2. physiological taxis chemotaxis