Definition of Alphabet
An alphabet is a complete standardized set of letters — basic written symbols — each of which roughly represents a phoneme of a spoken language, either as it exists now or as it may have been in the past. There are other systems of writing such as logosyllabic writing, in which each symbol represents a morpheme, or word or a syllable or places the word within a category, and syllabaries, in which each symbol represents a syllable.
The word "alphabet" itself is popularly believed to come from alpha and beta, the first two letters of the Greek alphabet, but some etymologists argue that instead the word derives from aleph and bet, the first two letters of the Phoenician alphabet (really a kind of syllabary) which later gave rise to the Hebrew alphabet. The true origin of the word is unclear. There are dozens of alphabets in use today. Most of them are 'linear', which means that they are made up of lines. Notable exceptions are Braille, manual alphabets, Morse code, and the cuneiform alphabet of the ancient city of Ugarit.
In spite of its imprecision, the term "alphabet" is commonly used to refer to any writing system whose graphemes represents both consonant and vowel sounds.
A grapheme is an abstract entity which may be physically represented by different styles of glyphs. There are many written entities which do not form part of the alphabet, including numbers, mathematical symbols, and punctuation. Some human languages are commonly written by using a combination of logograms (which represent morphemes or words) and syllabograms instead of an alphabet. Egyptian hieroglyphs and Chinese characters are two of the best-known writing systems with predominantly non-alphabetic representations.
Non-written languages also have alphabetic and non-alphabetic representations. For example, in American Sign Language one can spell words using the character set borrowed from the English language alphabet. Experienced ASL signers express most concepts using ideomatic hand signs which either correspond to English words or are original to the signed language.
Most, if not all human languages have some means for phonetic approximation of foreign words, usually using the native character set.