Victims of AoE spells are referred to as targets.
This isn't easy to find, but there is a rule in the PHB which specifically uses "target" to refer to someone affected by an AoE spell, in the section "Targeting Yourself" (emphasis mine):
If you are in the area of effect of a spell you cast, you can target yourself.
Additionally, we see in the rules for Saving Throws (emphasis mine):
Many spells specify that a target can make a saving throw to avoid some or all of a spell's effects. The spell specifies the ability that the target uses for the save and what happens on a success or failure.
AoE spells are obviously in view here. Further, in the DMG's rules for Adjudicating Areas of Effect, we see (p. 249-250; emphasis mine):
If you would like more guidance, consider using the Targets in Areas of Effect table. To use the table, imagine which combatants are near one another, and let the table guide you in determining the number of those combatants that are caught in an area of effect. Add or subtract targets based on how bunched up the potential targets are. Consider rolling 1d3 to determine the amount to add or subtract.
[There is a table here]
For example, if a wizard directs burning hands (a 15-foot cone) at a nearby group of orcs, you could use the table and say that two orcs are targeted (15 ÷ 10 = 1.5, rounded up to 2). Similarly, a sorcerer could launch a lightning bolt (100-foot line) at some ogres and hobgoblins, and you could use the table to say four of the monsters are targeted (100 ÷ 30 = 3.33, rounded up to 4).
In the rules for using miniatures on a combat grid, we see the following about areas of effect (p. 251; emphasis mine):
The area of effect of a spell, monster ability, or other feature must be translated onto squares or hexes to determine which potential targets are in the area and which aren’t.
For a more specific example, the spell descirption of fireball even calls creatures it affects "targets":
A target takes 8d6 fire damage on a failed save, or half as much damage on a successful one.
This should work with Area of Effect spells.
The Graviturgy Magic wizard's Gravity Well feature says (EGtW, p. 195; emphasis mine):
You’ve learned how to manipulate gravity around a living being: whenever you cast a spell on a creature, you can move the target 5 feet to an unoccupied space of your choice if the target is willing to move, the spell hits it with an attack, or it fails a saving throw against the spell.
If you cast a spell affecting one or more creatures, you have cast a spell on "a" creature, so you may select "a" creature, who becomes "the" creature affected by the Gravity Well ability.
Notably, many features explicitly state when they only work for spells that target a single creature, such as the sorcerer's Twinned Spell Metamagic option:
When you cast a spell that targets only one creature and doesn’t have a range of self, you can spend a number of sorcery points equal to the spell’s level to target a second creature in range with the same spell (1 sorcery point if the spell is a cantrip).
To be eligible, a spell must be incapable of targeting more than one creature at the spell’s current level. For example, magic missile and scorching ray aren’t eligible, but ray of frost and chromatic orb are.
At least to me, this suggests that if Gravity Well only worked for single-target spells, it would say so explicitly, rather than using an indefinite article.