Apparatus, n.; pl. Apparatus, also rarely Apparatuses.
Etymology: [L., from apparare, apparatum, to prepare; ad + prepare to make ready.]
1. Things provided as means to some end.
2. Hence: 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.
3. (Physiol.)
A collection of organs all of which unite in a common function; as, the respiratory apparatus.
Shades of meaning
Fernald has no entry of its own for apparatus; he lists it under tool.
Where he writes about it:
An apparatus may be a machine, but the word is commonly used for a collection of distinct articles to be used in connection or combination for a certain purpose--a mechanical equipment; as, the apparatus of a gymnasium; especially, for a collection of appliances for some scientific purpose; as, a chemical or surgical apparatus; an apparatus may include many tools, instruments, or implements. — from tool