A tutu is a dress worn as a costume in a classical ballet performance, often with attached bodice. It may be made of tarlatan, muslin, silk, tulle, gauze, or nylon. Modern tutus have two basic types: the Romantic tutu is soft and bell-shaped, reaching the calf or ankle; the Classical tutu is short and stiff, projecting horizontally from the waist and hip.

The word tutu can refer to only the skirt part of the costume. The bodice and tutu make up what is usually the entire costume, but which is called the tutu (by synecdoche, wherein the part – the skirt – can embody the whole).

The derivation of the word tutu is unknown. The word was not recorded anywhere until 1881. One theory is that it is simply derived from the word tulle (one of the materials from which it is made).^{[citation needed]}

A second theory is that the word comes from the slang of French children that refers to the buttocks (cul). During that era, the abonnés (rich male subscribers at the Paris Opera Ballet) were accustomed to mix with the ballet girls in the foyer and arrange assignations. It is suggested the expression came from the abonnés playfully patting the back of the tulle dress with the saying pan-pan cucul (French for I'll spank your bottom).^{[citation needed]}

A third, related theory suggests a derivation from the more vulgar French word cul (which can be used to refer to the bottom or genital area). During this era, women (including dancers) wore pantalettes as underwear, which were open at the crotch. The abonnés favoured the very front rows in the hope of a scandalous view, and the skirt was modified for that reason. This is supported by the description by nineteenth-century balletomane, Charles Nuitter, who defined tutu as "a slang term for the very short petticoat worn by danseuses in the interest of modesty."

According to Etimonline, tutu first appeared in 1910 and is derived from the French tutu, which in turn was derived from the French cucu, in turn derived from the French cul meaning a bottom or backside. Similarly, Merriam-Webster claims that the word first appeared in 1913 and that it is derived from the French tutu which is babytalk for a backside.

Marie Taglioni (1829) is believed to be the first person to sport the tutu, but the first designs of tutus actually originated two years before Taglioni performed on stage. Hippolyte Lecomte designed the basic silhouette of the Romantic tutu for Pauline Montessu in La Somnambule (1827); Taglioni's Sylphide costume's designs were not actually found. Ivor Guest points out the costume is very similar to what ballerinas would wear in class. The part of Creuse in Noverre's ballet Jason et Medée was danced by Mlle Guimard who wore a pale dress in 1770. The La Sylphide tutu may have been inspired by the use of muslin petticoats to give the skirt volume instead of using the usual hoops.

Towards the end of the 18th century, female dresses had higher waistlines and became slimmer as well; dancers appeared to begin dancing without panniers (hips hoops to accentuate skirt designs) for a more natural theme that displayed the human body and allowed more freedom in movements. These translated well in costumes in order to accent the lines of the dancers. Skin-colored tights were also worn with these evolved form-fitting costumes to preserve modesty, but replace the unseemly shapes of knickers.