Consider a scenario: you have just returned from the hardware store with two new 50-foot garden hoses with which you may now reach the 90 foot distance to the far corner of your yard. The casual observer sees you holding two appliances in your arms - two garden hoses.  But to us pipe and hose scientists, you are holding six appliances: two sections of flexible hose (sized by its internal diameter), plus four fittings (two on each hose section).  Each of the fittings has two fitments, and each section of hose has, itself, two very simple fitments.  So now let us reassert these foundational definitions:

APPLIANCE: For our purposes, an appliance is anything that something else attaches to, for example:

• a length of pipe
• a radiator
• a D-9 Caterpillar.

FITMENT: Any place two mechanical appliances meet and are connected mechanically for the purpose of fluid transfer, you have a fitment. Actually there are two fitments here, for example:

• NPT female and NPT male - both NPT
• Hose End (female), and Hose Barb (male)

FITTING: An appliance comprising an internal fluid flow channel and at least two fitments to which other appliances may be fitted.  These other appliances may each be any of the following:

• pipe
• hose
• tube or tubing
• manifold
• machine
• another fitting

This is the terminology used on this website. So everything can be categorized as an appliance, a fitment, or a fitting.  Note that a fitting is also an appliance, as is a pipe, a length of hose or tubing, etc.  Pretty much everything that can be held in your hand or moved onto your shop floor with a crane is an appliance.