The thong is a garment generally used as either underwear or in some countries, as a swimsuit. It may also be worn for traditional ceremonies or competitions.

Viewed from the front, the thong typically resembles a bikini bottom, while at the back the material is reduced to a minimum. Thongs are almost always designed to cover the genitals, anus, and perineum and leave part or most of the buttocks uncovered. The back of the garment typically consists of a thin waistband and a thin strip of material, designed to be worn between the buttocks, that connects the middle of the waistband with the bottom front of the garment. It is also used as a descriptive term in other types of garment, such as a bodysuit, bodystocking, leotard, or one-piece swimsuit, with the meaning "thong-backed".

One type of thong is the G-string, the back of which consists only of a (typically elasticized) string. The two terms G-string and thong are often used interchangeably; however, they can refer to distinct pieces of clothing. Thongs come in a variety of styles depending on the thickness, material or type of the rear portion of fabric and are used by both men and women throughout most of the world.

A tanga is a pair of briefs consisting of small panels connected by strings at the sides. There are tanga briefs both for men and for women. The style and the word come from Brazil where it is Portuguese for "thong".

The origin of the word thong in the English language is from Old English þwong, a flexible leather cord.

Many languages borrow the English word string to refer to this type of underwear, usually without the G. Another common name is tanga (or sometimes string tanga), especially in the German Tanga. A frequent metaphor, especially in Brazil, is dental floss; in Brazil a thong is called fio dental (Portuguese for 'dental floss'); in English, the term "Butt floss" is sometimes used. In Lithuanian it is siaurikės ('narrows'), in Italian perizoma or tanga, in Turkish ipli külot ('stringed underpants'), and in Bulgarian as prashka (прашка), which means a slingshot. In Israel the thong, mostly the G-string, is called khutini' (חוטיני), from the word khut, which means 'string'. Similarly, in Iran, it is called shortbandi (شورت بندی) in which short (شورت, from English: shorts) means 'briefs' and bandi means 'with a string'. A Puerto Rican Spanish slang term, used by Reggaeton artists, is gistro.^{[citation needed]}

Some names for the thong make reference to the bareness of the buttocks, such as the Spanish word colaless. (The word's origin is probably connected to the English term topless but in reference to cola, a colloquial word meaning 'butt' in South American Spanish.)^{[citation needed]} In some other languages the "T"-like shape of the back is emphasised. In Chinese, the thong is commonly called dingziku (丁字褲/丁字裤) which literally means '丁 character pants' (or roughly, 'T-letter pants'). In Korean, it is called 티팬티 (T panty). The term T-back is sometimes used in English, as in the novel T-Backs, T-Shirts, COAT, and Suit by E. L. Konigsburg.^{[citation needed]}

Colloquially, thongs and G-strings are often used interchangeably to describe skimpy underwear with minimal back coverage, although the main difference is usually attributed to the width of the strap in the rear. This is a definition reflected in the Encyclopedia of Clothing and Fashion, which considers the G-string or thong to be "a panty front with a half- to one-inch strip of fabric at the back that sits between the buttocks", using the terms interchangeably. Conversely, Knickers: a Brief History says: "Minor tweaks to the cut earned these skimpy panties different titles—from the thong, which has a one-inch strip of fabric down the back, to a G-string, which, as the name equivalent of Spanish suggests (hilo dental), is more like a string of fabric akin between the teeth."