A bathroom was a room for washing, containing devices such as a bath, shower, and/or toilet.
Both the Fourth Doctor and Leela referred to the TARDIS swimming pool as the "bathroom". (TV: The Invasion of Time)
On Earth, bathrooms were sometimes strictly gendered into separate men and women's facilities. According to Alicia, "men in the ladies loos" tended to be androgynous, or otherwise "sex-crazed" or looking confused. (PROSE: A Story of the Peace) The Bane Miss Wormwood bemoaned her subordinate Davey for hesitating to enter a girls' bathroom out of fear for the human culture which said males must not enter. (TV: Invasion of the Bane)
Blon Fel-Fotch Passameer-Day Slitheen, in the form of Margaret Blaine, took Cathy Salt to the ladies' in order to kill her, using the toilet while Cathy waited outside. When Cathy revealed she was pregnant, Margaret decided to let her go. (TV: Boom Town)
Both Penny Carter and Donna Noble hid in the bathroom for a day, while investigating Adipose Industries. Miss Foster soon discovered someone was spying on her and searched the toilet cubicles. She found Penny, but not Donna. (TV: Partners in Crime)
Amy Pond went to the ladies' bathroom of the White House in 1969 because she wasn't feeling well. She discovered a Silent there, and saw it destroy a woman called Joy. (TV: The Impossible Astronaut)
Airplanes also had bathrooms. Jack Harkness once visited one after being poisoned by arsenic in his cola. He vomited there. (TV: Rendition)
Evangelista was said to have confused an escape pod for a bathroom. (TV: Silence in the Library)