(noun)
a group of parts that function together (especially in a cell or some other anatomical context), or an appliance or instrument designed for some specific procedure;
e.g. This apparatus is involved in generating energy to power the system.
(noun)
the complex structures, including the functional processes that make up some system or organization, especially in a political or government context;
e.g. I trust that the apparatus of the law will ensure that justice is served.
(noun)
a collection of additional material, such as notes and alternative readings, that accompany some printed document;
e.g. I checked the apparatus of the book for clarification on a confusing point.