Cleavage

The cleavage was the area between a woman's breasts, especially when revealed by a low-cut neckline.

When possessing Rose Tyler's body, Cassandra O'Brien.Δ17 unbuttoned her blouse and exposed her cleavage in an attempt to make herself appear more desirable. She also took visible pleasure in concealing a sleep-inducing perfume down Rose's décolletage. Rose continued to wear the blouse this way without shame, including when she watched Cassandra pass on. (TV: New Earth)

Martha Jone's cleavage was shown with her tops. (TV: Smith and Jones, The Shakespeare Code, Gridlock, etc.)

Grace Holloway's cleavage was shown when wearing her opera dress. (TV: Doctor Who)

While making out with Gwen Cooper under the pheromone control of the Sex Gas, Carys Fletcher tore open Gwen's blouse. The sight of Gwen's exposed cleavage caused the gas inhabiting Carys to realise she was the wrong gender, and that only a man would provide her with the sexual energy it craved. (TV: Day One)

During her early travels with the Twelfth Doctor, Bill Potts wore a rainbow Henley top that showed off her cleavage. (TV: Smile)

Madame de Pompadour's cleavage was shown with every dress she wore. (TV: The Girl in the Fireplace)

River Song proudly displayed her cleavage in a series of outfits and dresses she wore during her travels. (TV: The Time of Angels, The Pandorica Opens, Day of the Moon, Lets Kill Hitler, The Angels Take Manhattan, The Husbands of River Song)

Melody Malone's cleavage "could fell an ox at twenty feet", according to Melody Malone: Private Detective in Old New York Town. (TV: The Angels Take Manhattan)