From the OED:
A pleasant, distinctive smell frequently accompanying the first rain after a long period of warm, dry weather in certain regions. Also: an oily liquid mixture of organic compounds which collects in the ground and is believed to be responsible for this smell.
What I'd like to know is, how have I, a poet who loves dirt and the weather, gone this long without knowing this word?
Many thanks to Jean Morris who posts at Tasting Rhubarb for enlightening me. Morris links to Marja-Leena Rathje's blog with more information. Apparently Bear and Thomas coined the word in 1964 for use in their article as quoted by the OED. Cool that the OED quote is the original use of the word.
Here's a little Van Gogh painting to imprint the word for me, "Wheat Field In Rain." (Click image for link to original website.)