Noun
A vowel which is normally pronounced somewhat longer than other vowels (usually around 1½ to 2½ times the length of a short vowel); represented in the IPA by the addition of a colon-like symbol (ː, triangular colon), such as /aː/.
(English pronunciation) Any of the vowels or diphthongs resulting from the Great Vowel Shift's effect on Middle English's original long vowels, and best exhibited in the names of the vowel letters A, E, I, O, U. In American lexicography their pronunciation is indicated by a macron, as, ē.
Source: en.wiktionary.orgA long syllable contains either a long vowel or a short vowel followed by a consonant as is the case in the word maktūbun which syllabifies as mak-tū-bun. Source: Internet
A long syllable (–) is a syllable that either has a long vowel, one or more consonants at the end (or a long consonant), or both. Source: Internet
Both katakana and hiragana usually spell native long vowels with the addition of a second vowel kana, but katakana uses a vowel extender mark, called a chōonpu ("long vowel mark"), in foreign loanwords. Source: Internet
Because the first letter in a vowel digraph sometimes says its long vowel sound, as in sail, some phonics programs once taught that "when two vowels go walking, the first one does the talking." Source: Internet
For example, the Vai syllabary originally had separate glyphs for syllables ending in a coda (doŋ), a long vowel (soo), or a diphthong (bai), though not enough glyphs to distinguish all CV combinations (some distinctions were ignored). Source: Internet
A syllable that is heavy because it ends in a long vowel or diphthong is traditionally called syllaba nātūrā longa ('syllable long by nature'), and a syllable that is heavy because it ends in a consonant is called positióne longa ('long by position'). Source: Internet