Multiple realities
(covers information from several alternate timelines)
A part-time job was a type of employment that only took up part of one's available work hours. Taking a part-time job to supplement the income derived from one's main career was called moonlighting.
In an alternate 1944, Alicia Travers sarcastically remarked that Sal and his friends only did loan sharking as a part-time job after the man claimed they were members of the Construction Workers Union. (ENT: "Storm Front")
In 2375, Chakotay was duped by a telepathic pitcher plant into believing he had been offered a part-time professorship of anthropology at Starfleet Academy. He assured Captain Kathryn Janeway of the job's part-time status after she expressed her desire not to lose him as a crewmember. (VOY: "Bliss")