An alphabet is a standard set of letters (basic written symbols or graphemes) which is used to write one or more languages based on the general principle that the letters represent phonemes (basic significant sounds) of the spoken language. This is in contrast to other types of writing systems, such as syllabaries (in which each character represents a syllable) and logographies (in which each character represents a word, morpheme, or semantic unit).

The Proto-Canaanite script, later known as the Phoenician alphabet, is the first fully phonemic script. Thus the Phoenician alphabet is considered to be the first alphabet. The Phoenician alphabet is the ancestor of most modern alphabets, including Arabic, Greek, Latin, Cyrillic, Hebrew, and possibly Brahmic. According to terminology introduced by Peter T. Daniels, an "alphabet" is a script that represents both vowels and consonants as letters equally. In this narrow sense of the word the first "true" alphabet was the Greek alphabet, which was developed on the basis of the earlier Phoenician alphabet. In other alphabetic scripts such as the original Phoenician, Hebrew or Arabic, letters predominantly or exclusively represent consonants; such a script is also called an abjad. A third type, called abugida or alphasyllabary, is one where vowels are shown by diacritics or modifications of consonantal base letters, as in Devanagari and other South Asian scripts.

Alphabet Inc. (commonly known as Alphabet, and frequently informally referred to as Google) is an American multinational conglomerate created in 2015 as the parent company of Google and several other companies previously owned by or tied to Google. The company is based in California and headed by Google's co-founders, Larry Page and Sergey Brin, with Page serving as CEO and Brin as President. The reorganization of Google into Alphabet was completed on October 2, 2015. Alphabet's portfolio encompasses several industries, including technology, life sciences, investment capital, and research. Some of its subsidiaries include Google, Calico, GV, Google Capital, X, Google Fiber and Nest Labs. Some of the subsidiaries of Alphabet have altered their names since leaving Google - Google Ventures becoming GV, Google Life Sciences becoming Verily and Google X becoming just X. Following the restructuring Page became CEO of Alphabet while Sundar Pichai took his position as CEO of Google. Shares of Google's stock have been converted into Alphabet stock, which trade under Google's former ticker symbols of "GOOG" and "GOOGL".

In formal language theory, a string is defined as a finite sequence of members of an underlying base set; this set is called the alphabet of a string or collection of strings. The members of the set are called symbols, and are typically thought of as representing letters, characters, or digits. For example, a common alphabet is {0,1}, the binary alphabet, and a binary string is a string drawn from the alphabet {0,1}. An infinite sequence of letters may be constructed from elements of an alphabet as well.

Given an alphabet , the set of all strings over the alphabet of length is indicated by . The set of all finite strings (regardless of their length) is indicated by the Kleene star operator as , and is also called the Kleene closure of . The notation indicates the set of all infinite sequences over the alphabet , and indicates the set of all finite or infinite sequences.

For example, using the binary alphabet {0,1}, the strings ε, 0, 1, 00, 01, 10, 11, 000, etc. are all in the Kleene closure of the alphabet (where ε represents the empty string).