Extra-shortness

The International Phonetic Alphabet (IPA) uses a breve ⟨ ˘ ⟩ to indicate a speech sound (usually a vowel) with 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 [pə̆ˈliˑs].^[1] This is typical of vowel reduction.

Extra-short
◌̆
IPA number	505
Encoding
Entity (decimal)	̆
Unicode (hex)	U+0306

Before the 1989 Kiel Convention, the breve was used for a non-syllabic vowel (that is, part of a diphthong), which is now indicated by an inverted breve placed under the vowel letter, as in eye [aɪ̯]. It is also sometimes used for any flap consonants missing dedicated symbols in the IPA, since a flap is in effect a very brief stop.

References

1. International Phonetic Association (1999). Handbook of the International Phonetic Association: A Guide to the Use of the International Phonetic Alphabet. Cambridge University Press. p. 23. ISBN 0521652367.