Lengthened vowel

IPA vowel length
◌ː ◌ˑ ◌̆
IPA number	503 or 504 or 505
Encoding
Entity (decimal)	ː​ˑ​̆
Unicode (hex)	U+02D0 or U+02D1 or U+0306

In linguistics, vowel length is the perceived duration of a vowel sound. Often the chroneme, or the "longness", acts like a consonant, and may have arisen from one etymologically, such as in Australian English. While not distinctive in most other dialects of English, vowel length is an important phonemic factor in many other languages, for instance in Arabic, Finnish, Fijian, Kannada, Japanese, Old English, Scottish Gaelic and Vietnamese. It plays a phonetic role in the majority of dialects of British English and is said to be phonemic in a few other dialects, such as Australian English, South African English and New Zealand English. It also plays a lesser phonetic role in Cantonese, unlike other varieties of Chinese.

Many languages do not distinguish vowel length phonemically. Those that do usually distinguish between short vowels and long vowels. A very few languages distinguish three phonemic vowel lengths, such as Luiseño and Mixe. However, some languages with two vowel lengths also have words in which long vowels appear adjacent to other short or long vowels of the same type: Japanese hōō "phoenix" or Ancient Greek ἀάατος [a.áː.a.tos] "inviolable". Some languages that do not ordinarily have phonemic vowel length but permit vowel hiatus may similarly exhibit sequences of identical vowel phonemes that yield phonetically long vowels, such as Georgian გააადვილებ [ɡa.a.ad.vil.eb] "you will facilitate it".

...
Wikipedia