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

noun. The orientation or locomotion of cells or of organisms in relation to the direction of the earth (or to gravity).

noun. A living creature's ability to move by orienting itself by gravitational forces.

"Sexual isolation caused by selection for positive and negative phototaxis and geotaxis in Drosophila pseudoobscura."

phototaxis  

Axis  axis