Object permanence

Etymology

The concept comes from developmental psychologist Jean Piaget, who observed in the 1930s–40s that infants under about eight months act as if a hidden object has vanished. “Object” from Latin “obiectum” (“thing thrown before” the mind); “permanence” from Latin “permanere” (“to remain throughout”). It has lately become internet slang for forgetting anyone you cannot currently see.

noun phrase · /OB-jekt PUR-muh-nunss/
The understanding that things continue to exist even when they cannot be seen, heard, or touched.
What it names
An early developmental milestone — and the metaphor now used for people or duties one forgets the moment they are out of sight.
Example
Babies delight in peekaboo precisely because they haven’t yet mastered object permanence.
Related idea
Object permanence is knowing the unseen still exists; out of sight, out of mind is the everyday failure to act on it, and it differs from memory itself, which is about recall rather than existence.