An abugida is a writing system that is neither a syllabic nor alphabetic script, but somewhere in between. It has sequences of consonants and vowels that are written together as a unit, each based on the consonant letter. Vowels must be written down as well, but they are secondary and do not act the same way consonants do.
Syllables are built up of consonants, each of which has an inherent vowel, which means a vowel is assumed to be pronounced in a syllable even if it is not written down. For example, in Thai, the syllable written as the consonant-vowel pair หา is pronounced haa, but the syllable just written as a consonant ห is pronounced hɔ.
Diacritic symbols may be used to change or mute the inherent vowel, and separate vowel letters may be used when vowels occur at the beginning of a syllable or on their own.
In some abugidas, certain vowels can be written to the right, to the left, on top, or below the consonant that is spoken before it, even if the reading order of the consonants must all be left-to-right or right-to-left. An example of this will use the Thai consonant ก to show where the following vowels must be written.
|Thai vowel letter||Pronunciation||syllable with ก||Pronunciation|
|า||IPA: [aː]||กา||IPA: [kaː]|
|ไ||IPA: [ai]||ไก||IPA: [kai]|
|ิ||IPA: [iː]||กิ||IPA: [ki]|
|ู||IPA: [u]||กู||IPA: [ku]|
This is different from a truly alphabetic script, in which the vowels and consonants have the same status, and an abjad, whose vowels can be left out. Ethiopic, most scripts of the Indian subcontinent, Mainland Southeast Asia, and some scripts of Maritime Southeast Asia and Canadian syllabics are abugidas.
The oldest known abugida is the Brahmi script, a writing system that was invented in modern-day India. Most abugidas that exist today, including Devanagari, Khmer, and Thai, were based on the Brahmi script. Abugidas based on Brahmi spread along with the Indian religions throughout the Indian subcontinent and Southeast Asia, especially Buddhism and Hinduism.
The name 'abugida' is derived from the first four letters of the Ge'ez (Ethiopic) alphasyllabary writing system.