Transgender

A transgender person was someone whose gender did not align with that which they were assigned at birth.

Mickey Smith had a trans friend called Sally Salter, who was part of his Bad Wolf band. (PROSE: Rose)

Cassandra O'Brien.Δ17 told Rose Tyler that she was once "a little boy" on Earth. (TV: The End of the World)

Jack Harkness once had a transgender co-worker, who went away and came back with a new name, Vanessa. (TV: Greeks Bearing Gifts)

Behind the scenes
In 2015, Bethany Black became the first openly trans actor to be cast in an episode of Doctor Who, series 9's Sleep No More. She played the role of 474.