Pele's hair (PAY-lays hair) noun
Thin strands of volcanic glass, formed when lava is thrown into the air by the explosion of a volcano.
[After Pele, the Hawaiian goddess of fire and volcanoes.]
I now know this thanks to one of the many nifty services from Wordsmith.org, so they get the official Monster HRTotM seal of, uhm, recommendation. It's a low-tech interface (be prepared to click around a little) but it's got a set of great cross-web expansions on a typical "a word a day" service.
There's just something about a site like this on today's web that makes me smile. Loads of info presented in a stripped down, no-frills manner. Just the simple premise... no goofy" Vocabularious!" podcast or "Libragarious", the social network for uppity English speakers.
Hmmm... I should check to see if libragarious.com is taken...