An alphabet is a writing system that uses a standard set of symbols, called letters, to represent particular sounds in a spoken language. Specifically, letters largely correspond to phonemes as the smallest sound segments that can distinguish one word from another in a given language. Not all writing systems represent language in this way: a syllabary assigns symbols to spoken syllables, while logographies assign symbols to words, morphemes, or other semantic units.

The first letters were invented in Ancient Egypt to serve as an aid in writing Egyptian hieroglyphs; these are referred to as Egyptian uniliteral signs by lexicographers. This system was used until the 5th century AD, and fundamentally differed by adding pronunciation hints to existing hieroglyphs that had previously carried no pronunciation information. Later on, these phonemic symbols also became used to transcribe foreign words. The first fully phonemic script was the Proto-Sinaitic script, also descending from Egyptian hieroglyphs, which was later modified to create the Phoenician alphabet. The Phoenician system is considered the first true alphabet and is the ultimate ancestor of many modern scripts, including Arabic, Cyrillic, Greek, Hebrew, Latin, and possibly Brahmic.

Peter T. Daniels distinguishes true alphabets—which use letters to represent both consonants and vowels—from both abugidas and abjads, which only need letters for consonants. Abugidas represent them with diacritics added to letters, while abjads generally lack vowel indicators altogether. In this narrower sense, the Greek alphabet was the first true alphabet; it was originally derived from the Phoenician alphabet, which was an abjad.

Alphabets usually have a standard ordering for their letters. This makes alphabets a useful tool in collation, as words can be listed in a well-defined order—commonly known as alphabetical order. This also means that letters may be used as a method of "numbering" ordered items. Some systems demonstrate acrophony, a phenomenon where letters have been given names distinct from their pronunciations. Systems with acrophony include Greek, Arabic, Hebrew, and Syriac; systems without include the Latin alphabet.

The English word alphabet came into Middle English from the Late Latin word alphabetum, which in turn originated in the Greek ἀλφάβητος alphábētos; it was made from the first two letters of the Greek alphabet, alpha (α) and beta (β). The names for the Greek letters, in turn, came from the first two letters of the Phoenician alphabet: aleph, the word for ox, and bet, the word for house.

The Ancient Egyptian writing system had a set of some 24 hieroglyphs that are called uniliterals, which are glyphs that provide one sound. These glyphs were used as pronunciation guides for logograms, to write grammatical inflections, and, later, to transcribe loan words and foreign names. The script was used a fair amount in the 4th century AD. However, after pagan temples were closed down, it was forgotten in the 5th century until the discovery of the Rosetta Stone. There was also cuneiform, primarily used to write several ancient languages, including Sumerian. The last known use of cuneiform was in 75 AD, after which the script fell out of use. In the Middle Bronze Age, an apparently alphabetic system known as the Proto-Sinaitic script appeared in Egyptian turquoise mines in the Sinai Peninsula c. 1840 BC, apparently left by Canaanite workers. Orly Goldwasser has connected the illiterate turquoise miner graffiti theory to the origin of the alphabet. In 1999, American Egyptologists John and Deborah Darnell discovered an earlier version of this first alphabet at the Wadi el-Hol valley. The script dated to c. 1800 BC and shows evidence of having been adapted from specific forms of Egyptian hieroglyphs that could be dated to c. 2000 BC, strongly suggesting that the first alphabet had developed about that time. The script was based on letter appearances and names, believed to be based on Egyptian hieroglyphs. This script had no characters representing vowels. Originally, it probably was a syllabary—a script where syllables are represented with characters—with symbols that were not needed being removed. The best-attested Bronze Age alphabet is Ugaritic, invented in Ugarit before the 15th century BC. This was an alphabetic cuneiform script with 30 signs, including three that indicate the following vowel. This script was not used after the destruction of Ugarit in 1178 BC.

The Proto-Sinaitic script eventually developed into the Phoenician alphabet, conventionally called Proto-Canaanite, before c. 1050 BC. The oldest text in Phoenician script is an inscription on the sarcophagus of King Ahiram c. 1000 BC. This script is the parent script of all western alphabets. By the 10th century BC, two other forms distinguish themselves, Canaanite and Aramaic. The Aramaic gave rise to the Hebrew alphabet.

The South Arabian alphabet, a sister script to the Phoenician alphabet, is the script from which the Geʽez script was descended. Abugidas are writing systems with characters comprising consonant–vowel sequences. Alphabets without obligatory vowels are called abjads, with examples being Arabic, Hebrew, and Syriac. The omission of vowels was not always a satisfactory solution due to the need of preserving sacred texts. "Weak" consonants are used to indicate vowels. These letters have a dual function since they can also be used as pure consonants.