Cleavage is the narrow depression or hollow between the breasts of a woman. The superior portion of cleavage may be accentuated by clothing such as a low-cut neckline that exposes the division, and often the term is used to describe the low neckline itself, instead of the term décolletage. Joseph Breen, head of the U.S. film industry's Production Code Administration, coined the term in its current meaning when evaluating the 1943 film The Outlaw, starring Jane Russell. The term was explained in Time magazine on August 5, 1946. It is most commonly used in the parlance of Western female fashion to refer to necklines that reveal or emphasize décolletage (display of the upper breast area).

In many cultures, though not all, men typically derive erotic pleasure from the visible display of cleavage. This tendency has been attributed to many reasons, including evolutionary psychology, a patriarchal revolution, and dissociation from breastfeeding. Since at least the 15th century, women in the Western world have used their cleavage to flirt, attract, make gender statements, and assert power. In several parts of the world, the advent of Christianity and Islam saw a sharp decline in the amount of cleavage which was considered socially acceptable. In many cultures today, cleavage exposure is considered unwelcome or is banned legally. In some areas like European beaches and among many indigenous populations across the world, cleavage exposure is acceptable; conversely, even in the Western world it is often discouraged in daywear or in public spaces. In some cases, exposed cleavage can be a target for unwanted voyeuristic photography or sexual harassment.

Cleavage-revealing clothes started becoming popular in the Christian West as it came out of the Early Middle Ages and enjoyed significant prevalence during Mid-Tang-era China, Elizabethan era England, and France over many centuries, particularly after the French Revolution. But in Victorian era England and during the flapper period of Western fashion, it was suppressed. Cleavage came vigorously back to Western fashion in the 1950s, particularly through Hollywood celebrities and lingerie brands. The consequent fascination with the cleavage was most prominent in the U.S., and countries heavily influenced by the U.S. With the advent of push-up and underwired bras that replaced corsets of the past, the cleavage fascination was propelled by these lingerie manufacturers. By the early 2020s, dramatization of the cleavage started to lose popularity along with the big lingerie brands. At the same time cleavage was sometimes replaced with other types of presentation of clothed breasts, like sideboobs and underboobs.

Many women enhance their cleavage through the use of things like brassières, falsies and corsetry, as well as surgical breast augmentation using saline or silicone implants and hormone therapy. Workouts, yoga, skin care, makeup, jewelry, tattoos and piercings are also used to embellish the cleavage. Male cleavage (also called heavage), accentuated by low necklines or unbuttoned shirts, is a film trend in Hollywood and Bollywood. Some men also groom their chests.