Nostalgia
Etymology
Coined in 1688 by the Swiss medical student Johannes Hofer as a Latinized Greek compound of “nostos” (homecoming) and “algos” (pain), to name the debilitating homesickness he observed in Swiss mercenaries far from home. Once a diagnosed illness, it gradually shed its medical status and broadened into the tender yearning for the past we mean today.
noun · /nos-TAL-juh/
A sentimental, wistful longing for the past.
What it names
The sweet ache of remembering a time or place you can revisit only in memory, and half wish you could return to.
Example
The smell of chalk dust filled him with nostalgia for a schoolhouse long since demolished.
Related idea
General longing can reach toward anything absent, including the future; nostalgia points specifically backward, coloured by the knowledge that what it wants is irretrievably gone.