1. The machines, tools, and equipment or machinery designed to serve a specific function for technical or scientific applications or that which is needed for a particular activity or purpose; for example, an X-ray apparatus.

A full collection or set of implements, or utensils, for a given duty, experimental or operative; any complex instrument or appliance, mechanical or chemical, for a specific action or operation; machinery; mechanism.

2. The complex structure of a particular organization or system or method by which an organization is operated or maintained; such as, an apparatus of government.
3. A group or system of body organs that collectively perform a specific function or process; such as, the respiratory apparatus, the digestive apparatus, or the vestibular apparatus (structures of the inner ear concerned with stimuli of equilibrium, including the semicircular canals, saccule, and utricle).
4. Etymology: from Latin apparatus, "tools, implements, equipment; preparation, a preparing" or "equipment which has been prepared for a particular use" or "purpose"; from the past participle stem of apparare "to prepare"; from ad-, "to" + parare, "to make ready".

A device, used in measuring gases, that absorbs a gas or some constituent from a mixture and estimates the quantity either from properties of the absorbed material or from the reduced residual volume remaining after absorption: "Mr. Dillman studied the operations manual of the absorption apparatus for three days before he conceded defeat."

The numerous body organs and structures consisting of hearing, especially the various parts of the ear: As future ear, eye, nose and throat specialists, some of the medical students found the acoustic apparatus to be the most difficult to master.