Medium

Fluid Conveyance Medium.
The MEDIUM is the thing you're using to transport the working fluid. The most common media are Pipe, Hose, and Tubing.

Fitment

A FITMENT is the mechanical configuration of the attachment end of a flow channel, for means of attaching to another medium or appliance. Examples of fitments include tapered thread, flare, compression, and many more.

Fitting

A FITTING is an appliance comprising an internal fluid flow channel and at least one fitment to which other appliances may be fitted. There are two-way fittings, three-way fittings, etc.  There are not many one-way fittings (only a cap and a plug come to mind right now). Examples of two-way fittings include nipple, coupling, union, elbow, etc. Examples of three-way fittings include tee, wye, and others.

Appliance

For our purposes, an APPLIANCE is anything that something else attaches to. We use the word appliance so that we don't have to say, "pipe or hose or fitting or whatever." Of course, an appliance could be a kitchen stove, but it could also be a length of pipe, or a D-9 Caterpillar.