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 syllable or places the word within a category, and syllabaries, in which each symbol represents a syllable.
The etymology of the word "alphabet" itself comes to Middle English from the Late Latin Alphabetum which in turn originates from the Ancient Greek Alphabetos, from alpha and beta, the first two letters of the Greek alphabet. 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, and Morse code.