What is a dubplate?

A dub plate is a one-off record. A dub is different to a regular vinyl record because instead of being pressed using a stamper, its cut real-time on a lathe. The lathe spins a blank disc, and the cutting head plays the audio and scores the groove in a spiral across the dub. There are two types: vinyl dubs and acetates, and they come in a range of sizes and layouts. See here for more info.