"Story arc" is a term which refers to a common thread in a series of stories, forming an overall "arc" throughout them. This can be a central subject that holds the stories together, such as Season 16 of Doctor Who, which featured the central theme throughout the season of the hunt for the Key to Time, or something less defined.
Throughout the novels published by Virgin Publishing and BBC Books, each of the main ranges featuring the Seventh Doctor and Eighth Doctor (respectively) had ongoing story arcs. During the Virgin New Adventures' first four novels, a common thread of a being known as the Timewyrm flowed throughout the first four novels.
Throughout the Virgin New Adventures there were several story arcs that grouped several novels around ideas that flowed through sets of novels. The novels Blood Heat, The Dimension Riders, The Left-Handed Hummingbird, Conundrum and No Future had a common idea and a central character revealed only in the final novel of the story arc as the individual responsible for events throughout the arc.
When Doctor Who was revived for television by BBC Wales, Russell T Davies, the show's executive producer and head writer, introduced the first story arc for the new series of Doctor Who, known now as the Bad Wolf arc. It was the defined story arc which appeared throughout the first series of BBC Wales' Doctor Who.
The story arc has become a common feature of the new televised Doctor Who, with each series revealing its own unique story arc carried throughout each series. The only series not to include a story arc within the revived era is eleventh series.