The fine powder collected from marijuana, usually from a grinder or pollen press or left at the bottom of a bag, consisting of crystals (trichomes), bud hairs, and small particles of plant material. Good quality keif always has a much higher thc (the main cannabinoid) content than marijuana bud itself. It tends to therefore induce stronger highs when smoked than marijuana.
I collected keif in the bottom chamber of my grinder, and topped a bowl with it. Now I'm so baked my eyes are receding.