Words have definitions for a reason but like so many things in the gun world, the meanings of certain words have become, shall we say, "elastic". Personally, I believe that a particular word's definition is written in stone, otherwise there is little point to having language at all. Here are some of the gun world's more flexibly defined words.
(actual meaning) - to return an item to its "as new" condition and configuration
(gun world meaning) - to sloppily refinish, usually including rounded corners and edges of metal and wood, smeared engraving, funneled screw and pin holes and incorrect finishes, often combined with half-assed repairs
"As new" is not better than new, nor is it what typically passes for a restoration by the vast majority of the practitioners in this business. A gun can only be correctly restored if the workman has detailed knowledge of how each part was prepped for finishing, the processes used at the factory for applying the finishes, the material the parts were made from and how they were heat-treated (if at all). No typical factory-made gun ever left the factory as a perfect specimen of gunmaking (not even close, typically). All factories tolerated less than perfect work in many aspects of the finished product. A correct restoration will show these same "mistakes", where appropriate. I did this type of work for a long time and am quite happy to say that I do it no longer.
For this one, the gun world meaning is really the only one that matters.
(gun world meaning) - to take a low or field grade gun and embellish/finish it in a way that, as closely as possible, mimics a factory-offered higher grade version of the same gun
That pretty much covers what an upgrade is as far as guns are concerned. Some however, might also refer to these as "custom" guns, which they most certainly are not, in any manner. I've never seen the point to expending the time and energy necessary to create an upgrade, mainly because I've never been particularly impressed with anything "high grade" offered by a factory, which is why I never have, and never will, create an "upgrade" gun. Slavishly copying something that already exists, especially something that was factory-produced, seems more than a bit unimaginative to me.
(actual meaning) - made, or done, to order for a particular customer, unique*
(gun world meaning) - any deviation from original, no matter how trivial
This is one that gets misused possibly even more than restoration. An AR-15, assembled with store-bought parts and accessories and spray-painted in a stars-and-stripes motif (I won't even get into flag etiquette)? Custom! A shotgun with crappy aftermarket engraving? Custom! Any "factory custom" that can be ordered out of the catalog? Custom! A Winchester Model 70 with an aftermarket stock? Custom! A "windowed" and stippled Glock? Custom!
None of the above examples actually qualify as a custom gun. They might qualify as "personalized" or "custom-ized" but nothing more. If Joe Sixpack can duplicate it with a Discover card and a Dremel, it probably doesn't qualify.
A true custom gun (or anything else) is by definition unique*.
*Unique: being the only one of its kind, unlike anything else
(actual meaning) - as new, like a freshly minted coin
(gun world meaning) - just about any condition, depending upon who is selling it
Actual mint-condition guns (old ones at least) aren't really that common. What is common is for people to refer to guns that are anywhere from 85-95 percent condition as "mint". These people are often dealers trying to justify an inflated asking price, or collectors trying to justify their paying an inflated asking price. Also, no matter the condition, ANY discernible repair automatically removes a gun from the mint category.
(actual meaning) - a record of ownership of a work of art or an antique, used as a guide to authenticity or quality
(gun world meaning) - a nebulous quality that causes an otherwise unremarkable gun to magically be worth many times its actual value, usually by virtue of once having been owned by a "celebrity"
The idea that any product's value increases due to once being owned by a celebrity has always puzzled me. Mere ownership by a celebrity creates no material change in the product and, in a rational world, would have no affect upon the item's value. For example, a Parker of whatever grade once owned by Clark Gable is still just a Parker of whatever grade. A beat-up Fox HE once owned by a racist outdoor writer unknown to most people, is still a beat-up Fox HE. These people had no hand in the creation of these items, they merely bought them, or the company gave them as "gifts" (a.k.a. advertising). Owning Clark Gable's Parker isn't going to transform you into Clark Gable, nor is it likely to make you a better shot.
That's it for now but I suspect that this list will grow (just a hunch).