gabbro (GAB-roh) - n., any of several dark, granular, plutonic, mafic igneous rocks chemically identical to basalt.
Which definition needs explication, or at least more definitions. Igneous means, of course, that it is solidified from molten rock coming up from deeper in the earth. Plutonic means that it solidified while beneath the surface of the earth -- in this case, if it had broken through to the surface, it would have become basalt as opposed to any of the other kinds of extrusive rocks. Mafic means it's a silicate rich in magnesium and iron (it's a portmanteau of magnesium and ferric). Granular means it's course-grained enough you can see individual crystals as grains, the size of which indicates how slowly it cooled (the larger, the slower). Dark is left as an exercise for the reader's imagination. The word was borrowed around 1830 from Italian, specifically Tuscan dialect, from Latin glaber, smooth. As for the word, I just happen to really like the sound.
Gabbro is to basalt as granite is to rhyolite.