noun.
The characteristic movement or orientation of an organism or cell along a chemical concentration gradient either toward or away from the chemical stimulus.
The Century Dictionary and Cyclopedia
noun.
The locomotion of organisms or of cells in relation to chemical substances, or the property of certain chemical substances to attract or repel living cells at the point of action: in the first instance there is positive chemotaxis, in the second negative chemotaxis. The peculiar response of the white blood-corpuscles to chemotactic stimulation is of fundamental importance in the defense of the animal organism against bacterial invasion.
the GNU version of the Collaborative International Dictionary of English
The sensitiveness exhibited by small free-swimming organisms, as bacteria, zoöspores of algæ, etc., to chemical substances held in solution. They may be attracted (positive chemotaxis) or repelled (negative chemotaxis).