I'm wondering if there's some more formal definition we can go to, beyond something like it's covering most of the body or is designed for long term combat. There's needs to be something more definite to it.
Nope, unless you go with JoeNapalm's "it's the one called Full-body armor in the rule book" (which there IS some support for, more on that later), there's only references to it in other rules.
Namely:
1) It's difficult to perform First Aid through. (possibly implying it's bulky or difficult to remove)
2)
IF it includes a helmet, it can take a chem seal. (implying that not all FBA has a helmet)
3) It cannot be combined with supplemental armor, PPP specifically. (implying it's either bulky or heavily armored enough that extra plating is redundant)
How about "it has to be capable of covering all of it"? Everything I named falls under that.
It's very open to interpretation and can be a bit of a slippery slope.
There are a few on your list that don't have 100% coverage, and you're missing a couple others that fit that definition.
There are only 4 kinds of body armor that come will full-helmets: FBA, Mil-Spec, Sec-Spec and Swat.
The rest have standard helmets. The illustration for riot gear shows a standard open faced helmet with a flip up face guard, for example.
Likewise, while the illustration for shows some sort of helmet/respirator/gas mask not mentioned in the armor's description, the armor is also depicted as lacking gloves (unless the gloves have fingernails).
Aside from space suits, that pretty well obviously have to be sealed. Only the drysuit, FBA and mil-spec armor make explicit mention of being hermetically seal-able. (this is the support I mentioned earlier for JoeNapalm's statement, why call it out if chem seal was so broadly available)
Finally, chameleon/sneak suits are typically depicted with a pull-over hood and facemask (which makes sense for an active camouflage system), so they'd technically fall under the "full coverage" rule.
I don't necessarily disagree with the suggestion, it certainly explains why so many types of armor list a standard helmet for the standard price (i.e. it's the only way to supplement the armor value), but rules 1 and 3 are fairly harsh restrictions new to 5th that, coupled with the relatively light protection of some of these pieces, I'm concerned about playability/balance shifting a slightly sub-optimal stylistic choice (bike racing armor, for example) to a rather harsh and incongruous one. Bunker gear (which, I'd like to point out, I DID say includes pants) should be no more difficult to proved first aid through than an armored jacket, for example.