A "Jason" dive is a term popularized by scuba divers in the Monterey Bay area. This neologism was coined by a dive shop manager in response to one of his employee's habit of completing only one scuba dive during an outing, even though the typical practice is to complete two scuba dives (a two tank dive).
Over the past year the term has caught on with local dive clubs, who announce single dive events as a "Jason." Some have interpreted it to mean cancelling a planned second dive, though it more appropriately refers to a planned single dive.
Diver A: Wanted to do a two tank dive, but I am going to have to pull a Jason today.