-taxis Also -taxy, -taxia, -tactic, and -taxic.
Arrangement or order; movement in response to an external stimulus.
[Greek taxis, orientation, arrangement.]
The senses of -taxis are derived from those of the free-standing word taxis.
Examples in the first sense include phyllotaxis (Greek phullon, leaf), the arrangement of leaves on an axis or stem, and stereotaxis (Greek stereos, solid), the use of medical devices that accurately place probes within the body. In linguistics, taxis can also refer to the arrangement of linguistic items in sequence, as in parataxis (Greek para-, beside), the placing of clauses or phrases one after another without words to indicate coordination or subordination.
Examples in the second sense include geotaxis, the motion of a motile organism or cell in response to the force of gravity; phototaxis, movement in response to light; and thigmotaxis (Greek thigma, touch), motion or orientation in response to touch.
Words in -taxy derive from the same source, but are equivalent to those ending in -taxis only in the sense of arrangement: phyllotaxy; stereotaxy; epitaxy, the growth of crystals on a crystalline substrate that determines their orientation. With the exception of epitaxy, forms in -taxy are usually less common than their counterparts in -taxis. Ataxy is a variant form of ataxia, the medical term for a loss of full control of bodily movements (literally ‘without order’, at first referring to an irregularity of function); terms in -taxia are otherwise rare.
Adjectives are commonly formed from these nouns using -tactic: geotactic, hypotactic, paratactic, stereotactic. A few also occur in the less common spelling -taxic: stereotaxic, thermotaxic, of an organism that moves in response to changes in temperature. However, ataxia has ataxic as its usual adjective.
One noun that does not belong with this set is epistaxis, bleeding from the nose, which is from Greek epi, upon, in addition, plus stazein, to drip.