A lava lamp is a type of lamp that contains an oil-based wax compound surrounded by a semi-viscous liquid. When the wax is heated by the bulb at the base, the wax lowers in density, therefore rising above the denser liquid. When it gets farther away from the heat source, it cools and becomes denser, therefore falling closer to the heat source. This repeats until the bulb is either burned out or the lamp is turned off.
Rain Robinson had a lava lamp in the Griffith Observatory. (VOY: "Future's End")