In school I learned that allusions are indirect references.
Often my teachers would give examples in the form of references to well-known literary works.
If I say "I can read the writing on the wall", I'm making a reference to the hand of God writing a prophesy of doom at the feast of Belshazzar in the book of Daniel, and comparing it to my own situation.
Or if I throw you a can of Sprite and say "here, obey your thirst", I'm alluding facetiously to the Sprite commercials from the late 90s.
These are the type of indirect references that I'm sure are allusions. But if the purpose of allusion as a poetic/rhetorical technique is to make the reader feel kinship through referencing the things we all know about, then I'm curious whether other indirect references would qualify as allusions.
A couple examples I'm curious about:
Isn't it romantic? / Moving shadows write / The oldest magic word
—"Isn't It Romantic" by Rodgers & Hart
"The oldest magic word" is referring indirectly, and via a metaphor, to love. We all know that love is ancient and magical. Would this qualify as an allusion?
"I know when that hotline bling / That can only mean one thing"
—"Hotline Bling" by Drake
The "one thing" that Drake's phone ringing late at night can mean is that the girl he's talking about is DTF. We understand what he's referring to and he doesn't have to say it directly. Does this qualify as an allusion?