D&D 5e is a bit ambiguous with its terms, so there is probably never going to be a comprehensive answer

The linked podcast is probably the best source of Rules as Intended we'll get. It says that "target" should be used in its natural English language meaning. This means that anything affected by a spell can be considered a target. I'm going to use lower-case target for this kind of meaning in the rest of this post.

The other main meaning is "the thing you choose as a target" according to some rule (such as a spell's text). This is a semi-formalized meaning, and Allan Mills' answer quotes some of the rules for this kind of targeting for spells. But elsewhere, the formal terminology blends in to the other less formal meaning a lot. I'm going to use all-caps TARGET for this meaning.

The TARGET meaning is definitely not the only one used in the rules, as spells like Fireball have TARGET you choose (a point in space), but may also effect a number of creatures (and flammable objects) in the area of effect. Affected creatures are are explicitly described as targets in the spell's rules (emphasis added):

Each creature in a 20-foot-radius sphere centered on that point must make a Dexterity saving throw. A target takes 8d6 fire damage on a failed save, or half as much damage on a successful one.

Unfortunately, some spells make it hard to judge who or what all the targets are. As the linked question you've provided show, it's not very clear if Warding Bond or Life Transference (from Xanathar's Guide to Everything), which both have the capacity to damage the caster, should treat the caster as a target or not. By the the logic in the podcast, the caster is affected and so must be a target. But they're definitely not the chosen TARGET, and it's possible that some parts of some rules (like the one that terminates Warding Bond early if either character has the spell cast on them again) may only want to refer to the character being the chosen TARGET of a future casting.

Another area where this comes up is teleportation spells. As the formal rules for picking spell TARGETs say, you normally need "a clear path to a TARGET":

To target something, you must have a clear path to it, so it can't be behind total cover.

One surprising consequence of this (as the podcast discusses) is that you can't cast spells through glass windows, since they count as total cover (personally I'd interpret cover relative to the type of effect, so a window would not provide cover to a non-physical effect like Hold Person, but it would indeed cause a Fireball to explode prematurely, though the blast would destroy the window and spread the AOE outside). But most teleportation spells are phrased such that you don't explicitly TARGET your destination, even though your arrival will clearly affect the place. So it's unclear if you need to a clear path to the destination or not. For some spells like Dimension Door and Teleport, it's pretty obvious that you're not expected to have a clear path, since they have specific rules for how you pick your destination even if you can only describe it or have had it described to you (an example given in the rules: "upward to the northwest at a 45- degree angle, 300 feet"). But other spells like Misty Step specifically say you need to see your destination, which makes it ambiguous if you can use them to teleport through transparent total cover (e.g. a window).

Ultimately, these ambiguities are design problems that the creators of D&D have not yet fixed (and they may never do so). As such it's up to each DM to make a ruling for their own table any time they come up. It is a clear design principle of 5e that the DM should be empowered to interpret the rules as best works for their game, and that the rules do not attempt to be entirely comprehensive.