The International Phonetic Alphabet uses a breve, [ ̆ ], to indicate a speech sound (usually a vowel) with less than normal or extra short duration. That is, [ă] is a very short vowel with the quality of [a].
An example from English is the short schwa of the word police [pə̆liˑs]. This is typical of vowel reduction.
For typographic convenience, the breve is sometimes used for a non-syllabic vowel (that is, part of a diphthong), which is officially indicated by a similar diacritic placed under the vowel letter, as in eye [aɪ̯]. It is also sometimes used for flap consonants which do not have dedicated symbols in the IPA, since a flap is in effect a very brief stop. However, the diacritic is may be found either on the symbols for stops, or on approximants or fricatives such as ⟨w̆⟩, ⟨v̆⟩, or ⟨ʟ̆⟩.