From Destinypedia, the Destiny wiki
Comets were planned expansion packs for the Destiny franchise. They were intended to bridge the gaps between the releases of the main installments of the series, and would have included more substantial content than a normal expansion. Comets were expected to be released in 2015, 2017, 2019 and 2021.
To date, Destiny has received four expansions and one update: The Dark Below, House of Wolves, The Taken King (the April Update) and Rise of Iron. While The Taken King fits the description of a Comet and was even called "Comet: Plague of Darkness" during an early stage of development, it was never identified as one in its pre-release marketing. In late 2013, the Comet that became The Taken King had an entirely different direction: it would have added Europa and the European Dead Zone as new locations. Those locations were dropped in March 2014 in favor of the Dreadnaught and an unidentified area on Mars. The Mars area was later cut for inclusion in Destiny 2, while the remainder of the Comet focused solely on the Dreadnaught. It was presumably during this stage of development that The Taken King became known as an expansion rather than a Comet. It is unlikely any future expansions will be called "Comets", as the moniker appears to have been discarded entirely by Bungie.