Wiktionary
n. A new tune that has been pressed to vinyl or acetate in very limited numbers.
Wikipedia
A dubplate is an acetate disc – usually 10 inches in diameter – used in mastering studios for quality control and test recordings before proceeding with the final master, and subsequent pressing of the record to be mass-produced on vinyl. The " dub" in dubplate is an allusion to the plate's use in "dubbing" or "doubling" the original version of a track. The name dubplate also refers to an exclusive, 'one-off' acetate disc recording pioneered by reggae sound systems but also used by drum and bass and other electronic music producers, DJs and sound systems.
These dubplates will often be either unreleased recordings (which may or may not end up being made available to the general public) or exclusive versions or remixes of existing recordings. They are often used as a market research tool to assess the probable sales of a tune once it is released, as they are far cheaper to produce than a pressed vinyl record. However, because the material of the disc is very soft, the groove is worn out little by little with every playback. After about fifty plays the loss in sound quality becomes noticeable.
Vinyl dubplates are a recently developed format which allow extremely durable recordings to be made (lasting 90% as long as pressed vinyl) and are more suited to cases where no release is imminent, or the release date is a long time away.