tropism

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. The turning or bending movement of an organism or a part toward or away from an external stimulus, such as light, heat, or gravity.

Century Dictionary and Cyclopedia

1. n. In biology, growth, bending, orientation, or locomotion of organisms, or of parts of organisms, in relation to external agents. Positive tropism is growth or motion toward and negative tropism away from the external agent. Tropisms are physiological reactions called forth by an agent working locally and resulting in a definite orientation. In this wide sense tropism includes taxis, and the various terms designating forms of tropism (see extract under def. 2) are often used where strict usage would require the corresponding term ending in -taxis.
2. n. In a stricter use, the growing or bending of organisms, or of parts of organisms, in relation to external agents. Tropisms are reactions to agents that act from one side and produce modifications of direction of growth. Different forms of tropism are designated, not by the differences in the organs that are stimulated, whether sense-organs or simple protoplasm, but by the nature of the exciting agent. See the extract.

Wiktionary

1. n. biology the turning of an organism in response to a stimulus, either towards or away from the stimulus
2. n. virology viral tropism, or which type of tissue supports a certain virus

GNU Webster's 1913

1. n. (Physiol.) Modification of the direction of growth, caused by some external influence, such as light; -- sometimes used for motion of an organism toward or away from an external stimulus, more properly called taxis.

WordNet 3.0

1. n. an involuntary orienting response; positive or negative reaction to a stimulus source