This word describes the scent of rain on dry earth. The word is constructed from Greek, petra, meaning stone + ichor, the fluid that flows in the veins of the gods in Greek mythology.
The term was coined in 1964 by two Australian researchers, Bear and Thomas, for an article in the journal Nature, where the authors describe how the smell derives from an oil exuded by certain plants during dry periods and is absorbed by clay-based soils and rocks. During rain, the oil is released into the air along with another compound, geosmin, producing the distinctive scent. Now you have a name for that great aroma.