geotaxis

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Movement of a motile organism using the earth's gravity for orientation.

Century Dictionary and Cyclopedia

1. n. The orientation or locomotion of cells or of organisms in relation to the direction of the earth (or to gravity). For example, “cockroaches seem to be stimulated by gravity when this acts perpendicularly to their ventral surface, so that they tend to move off from a horizontal surface and do not come to rest until they are on a more or less nearly vertical one.” C. B. Davenport, Exper. Morphol., p. 118.

Wiktionary

1. n. biology A living creature's ability to move by orienting itself by gravitational forces.