Tzatziki (Greek: τζατζίκι [d͡zaˈd͡zici]), also known as cacık (Turkish: [dʒaˈdʒɯk]) or tarator, is a class of dip, soup, or sauce found in the cuisines of Southeastern Europe and West Asia. It is made of salted strained yogurt or diluted yogurt mixed with cucumbers, garlic, salt, olive oil, red wine vinegar, sometimes with lemon juice, and herbs such as dill, mint, parsley and thyme. It is served as a cold appetiser (meze), a side dish, and as a sauce for souvlaki and gyros sandwiches and other foods.

The word tzatziki appeared in English around the mid-20th century as a loanword from Modern Greek (τζατζίκι), which in turn comes from the Turkish word cacık. The root is likely related to several words in West Asian languages. Persian zhazh (ژاژ) refers to various herbs used for cooking, and Kurdish jaj or ژاژ refers to the caraway herb. That word is combined with the Turkish diminutive suffix -cık to yield cacık. It may be related to an Armenian word, cacıg. According to Sevan Nişanyan the Armenian word may itself come from Turkish or Kurdish.

Evliya Çelebi's 1665 travelogue, the Seyahatnâme, defined cacıχ (cacıg) as a kind of herb that is added to food. The modern term cacık (جاجیك) was mentioned in print for the first time in 1844 in Hoca Kâmil Pasha's Melceü't-Tabbâhîn (The Sanctuary of Cooks), the first Ottoman cookbook, in which the basic description is given as "yogurt with cucumber and garlic" (hıyar ve sarmısaklı yoğurt).

The form tarator, found in languages from the Balkans to the Levant with derivative forms now found in a range of countries.

According to Sevan Nişanyan, the name tarator (sauce) comes into Turkish directly from the Venetian word "trattor". In Venetian, this word means "cook". It has acquired the meaning of a type of sauce in Turkish. It has likely been borrowed from Turkish into other languages. The first use of this word in the sense of sauce was in the book Seyehatname by the Turkish traveler Evliya Çelebi.

Tzatziki and similar yogurt-based dishes trace their origins to the Central Asian yogurt culture, which spread to Anatolia, the Balkans, and the Middle East through Turkic migrations. Turks had consumed yogurt since ancient times and introduced yogurt-based cold soups and sauces to Anatolia.

During the Ottoman Empire, yogurt and garlic condiments became a part of the meze tradition. One notable example was tarator, prepared with yogurt, cucumber, garlic, or sometimes walnuts. The 17th-century Ottoman traveler Evliya Çelebi, in his Seyahatname, described dishes made with yogurt and garlic, demonstrating their established place in Ottoman cuisine.

In the Balkans and the Levant, variations of tarator developed with different bases: in the Levant it is usually made with tahini, while in the Balkans it may include yogurt and walnuts. In Anatolia, however, the version made with yogurt, cucumber, garlic, and sometimes walnuts became known as cacık.