(anatomy) a group of body parts that work together to perform a given function
"the breathing apparatus"
The entirety of means whereby a specific production is made existent or task accomplished.
"The entire apparatus required to film the documentary included specialized cameras, lighting rigs, and editing software."
In plain English: An apparatus is any piece of equipment used for a specific purpose, such as tools for science experiments or sports gear.
"The rescue team used a complex apparatus to lift the stranded climbers safely out of the canyon."
Usage: Use apparatus to refer specifically to the complex equipment, machinery, or systems required to perform a particular scientific experiment or technical operation rather than general tools. Avoid using it for simple hand-held instruments where "device" or "tool" would be more natural in everyday conversation.