Cosplay was the act of dressing up as fictional characters.
The Eleventh Doctor visited a Doctor Who convention in a parallel universe in which his life was a television series, at which his fans were impressed at what they believed to be a very convincing cosplay of himself. Despite this, he still came second in the costume contest. (COMIC: The Girl Who Loved Doctor Who)
When the Twelfth Doctor and Clara landed at San Diego Comic-Con, two cosplayers were dressed as Doc Brown and Marty McFly, while a child was cosplaying as the Star Wars character Boba Fett. (COMIC: Selfie)
Petronella Osgood was interested in cosplay. Among the items on her computer desktop was a folder for cosplay, represented with an icon of a fez. (WC: The Zygon Isolation) A tab in her Chrome window was titled 'Cosplay Costumes at C'. (WC: The Zygon Isolation)
Behind the scenes
Cosplay was a significant activity within Doctor Who fandom in which people dressed up as characters from the show, typically at conventions, but potentially also at Halloween, fancy dress parties, or even for no particular occasion at all.
Fans' level of participation varied, from people who bought off-the-peg, pre-made costumes, to those who paid others to manufacture bespoke costumes, to those who designed and crafted unique items from initial sketches to finished product.
Starting with DWM 522, Christel Dee gave cosplay tips for costumes of several characters, such as Ace's jacket, the Doctor's scarf and a Time Lord collar.