Glossary | Telescope accessories | Equipment | DC power plug
This means a plug for the cigarette lighter socket in a car.
Most cars have a 12V socket for connecting external devices to the on-board power supply.
Generally, portable batteries, known as powertanks, have a DC power plug connection. Since powertanks are a very popular way of supplying power to an astronomical mount, a cable with a DC power plug is often included in the scope of delivery, or is at least available as an optional accessory.