Most of us operate under Part 91 of the FARs. One of the requirements to operate under instrument  conditions is that the aircraft must have certain  minimum equipment as specified by FAR 91.205. This section contains a list that includes things such as Airspeed Indicator, Altimeter, Tachometer, Artificial Horizon etc. These seem like generic terms but they carry a specific meaning to the FAA and in the context of the FARs.

Suppose I know nothing about making instruments yet I want to sell instruments to airplane builders. I look in the dictionary under Altimeter and see that it is "an instrument showing altitude". Armed with this knowledge I go outside and find a fist sized rock and bring it to my laboratory. I then reference a topographical map that shows the altitude of my laboratory to be 2,681 feet above sea level. I then measure the distance from the floor to the bench. It is three feet so I add this to 2,681 and, using an indelible marker, write "2,684 FEET ABOVE SEA LEVEL" on the rock. I take a picture of my work and place an ad  on ebay for sale as a "Calibrated Altimeter. Accurate to within 6 inches at 2,684 feet ASL. Accuracy varies with altitude. $10".

Why not???

When the FAA says "altimeter" they have a very specific thing in mind. That "thing" is defined in the Technical Standards Orders (TSOs). From the FAA:

"A Technical standard Order (TSO) is a minimum performance standard issued by the Administrator for specified materials, parts, processes, and appliances used on civil aircraft."

As you can imagine there are a lot of TSOs and they cover just about everything. In order to have an "altimeter" in your airplane you must have an "altimeter" as defined by the FAA. In order to be recognized by the FAA as an "altimeter" your "instrument showing altitude"  must have demonstrated that it conforms to TSO-C10b. If it does not then, in the eyes of the FAA, it might as well be a rock.

You can put all the fancy glass you want  on you panel BUT you must have a minimum set of instruments, as defined  in 91.205, that have demonstrated that they conform to their respective TSOs. You can "Experiment" all you want but if you want to "Operate" you have to follow the rules.

Regards
Brent Regan