ap·pa·rat·us 
(ăp′ə-rătəs, -rātəs)
(ăp′ə-rătəs, -rātəs)n. pl. apparatus or ap·pa·rat·us·es
1.
a. An appliance or device for a particular purpose: an x-ray apparatus.
b. An integrated group of materials or devices used for a particular purpose: dental apparatus. See Synonyms at equipment.
2.
a. The totality of means by which a designated function is performed or a specific task executed, as in a system of government.
b. A political organization or an underground political movement. Also called apparat.
3. Physiology A group or system of organs that collectively perform a specific function or process: the respiratory apparatus; the digestive apparatus.
4. The critical and source material, usually including notes and a glossary, that accompanies a scholarly edition of a text.
[Latin apparātus, preparation, from past participle of apparāre, to prepare : ad-, ad- + parāre, to prepare; see perə-1 in the Appendix of Indo-European roots.]