What is technically considered an attack?

[greenstone]

Uncanny Dodge is a great example of why this question matters. If you subscribe the the idea that any action that does damage is an attack, this becomes an incredibly powerful ability!

There are lots of such questions.

Does a rogue or ranger's uncanny dodge feature halve the damage of a dragonborn's breath weapon?

Does a dragonborn's breath weapon count for maintaining barbarian rage? For a warlock's hurl through hell feature?

Does magic missile qualify for a warlock's hurl through hell feature? If it does, spending a level 1 spell slot to do an automatic 10d10 psychic damage is pretty overpowered.

Does a druid's nature's sanctuary feature protect you against a winter wolf's breath weapon?