A type of AC power supply similar to a wall wart, except the transformer is connected to two cables, one of which plugs into the wall outlet and the other plugs into the piece of gear being powered. In some cases, one or both of these cables are detachable from the transformer.
A lump in the line has similar advantages over an internal power supply to a wall wart (low cost, ease of manufacture, versatility, reduced development cost, one-time UL approval, and sonic improvements such as reduced noise), but requires only one space on a power strip (unlike many wall warts, which can require two or three spaces) and doesn’t tend to fall out of a wall outlet or vertical power strip.