Sub-style of metal. Sometimes also known as "doom metal" due to its often agonizingly slow pace, crushing heaviness, and gloomy lyrics. It's generally accepted to have been invented by Black Sabbath. Some stoner metal bands are retro-oriented, some aren't. Common elements include marijuana references (obviously), sci-fi/fantasy themes, extended instrumental jams, and tempos anywhere from plodding dirges to medium-paced rock (but rarely faster). Heavy-ass guitar riffage is essential.
Notable bands include Kyuss, Sleep, Fu Manchu, Monster Magnet, Electric Wizard, Orange Goblin, etc.
Stoner metal is cool, but I don't like the retro aesthetic a lot of bands use.