Geosmin
Etymology
Coined in 1965 from Greek "geo-" ("earth") and "osme" ("smell"). It is produced mainly by soil bacteria called actinomycetes; the human nose can detect it at astonishingly low concentrations.
noun · /jee-OZ-min/
The organic compound responsible for the earthy smell of soil and of rain after a dry spell.
What it names
The exact molecule behind "the smell of rain" and fresh dirt — and the muddy off-taste in some water and beets.
Example
After the first storm in weeks, the yard filled with the sharp green scent of geosmin.
Related idea
Petrichor is the poetic name for the whole smell of rain on dry earth; geosmin is the specific chemical that gives that scent its earthy core.