Definition of "long vowel"

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/.  examples

(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.  examples