1- Style of instrumental reggae where the vocals are cut off via engineering work. Later evolved into straight up instrumental reggae.
2- Dubplate, Dub. First physical form of a drum and bass or jungle track, most likely on a cheap vinyl, for club testing purposes. Hence a dubplate is an unreleased work until it is put out by a label, thus becoming a proper record. Now applies to various dance|vinyl releases.
3- Second coming of dub (mostly minimal technoish). Features original dub techniques in its sound (prominent sub bass, delay, reverb, analog equipment). Think Basic Channel, Pole etc. (visit Ishkur's guide to electronic music for further examples)
4- Nowadays, it somewhat refers to an instumental of a track containing vocals. for example : Fedde le Grand - Put your Hands Up for Detroit (dub)