Waw (wāw "hook") is the sixth letter of the Semitic abjads, including Phoenician wāw 𐤅, Aramaic waw 𐡅, Hebrew vav ו‎, Syriac waw ܘ and Arabic wāw و‎ (sixth in abjadi order; 27th in modern Arabic order). It is also related to the Ancient North Arabian 𐪅‎‎‎, South Arabian 𐩥, and Ge'ez ወ.

It represents the consonant [w] in classical Hebrew, and [v] in modern Hebrew, as well as the vowels [u] and [o]. In text with niqqud, a dot is added to the left or on top of the letter to indicate, respectively, the two vowel pronunciations.

It is the origin of Greek Ϝ (digamma) and Υ (upsilon); Latin F, V and later the derived Y, U and W; and the also derived Cyrillic У and Ѵ.

In Hebrew, the word וָו vav is used to mean both "hook" and the letter's name (the name is also written וי״ו), while in Syriac and Arabic, waw to mean "hook" has fallen out of use.

The Arabic letter و is named واو wāw and is written in several ways depending on its position in the word:

Wāw is used to represent four distinct phonetic features:

As a vowel, wāw can serve as the carrier of a hamza: ؤ.

Wāw is the sole letter of the common Arabic word wa, the primary conjunction in Arabic, equivalent to "and". In writing, it is prefixed to the following word, sometimes including other conjunctions, such as وَلَكِن wa-lākin, meaning "but". Another function is the "oath", by preceding a noun of great significance to the speaker. It is often literally translatable to "By..." or "I swear to...", and is often used in the Qur'an in this way, and also in the generally fixed construction والله wallāh ("By Allah!" or "I swear to God!"). The word also appears, particularly in classical verse, in the construction known as wāw rubba, to introduce a description.