The Brønsted–Lowry definition is the most widely used definition; unless otherwise specified, acid–base reactions are assumed to involve the transfer of a proton (H+) from an acid to a base.
ðə bɹˈɑːnstᵻd lˈoʊɹi dˌɛfɪnˈɪʃən ɪz ðə mˈoʊst wˈaɪdli jˈuːzd dˌɛfɪnˈɪʃən; ʌnlˈɛs ˈʌðɚwˌaɪz spˈɛsᵻfˌaɪd, ˈæsɪd bˈeɪs ɹɪˈækʃənz ɑːɹ ɐsˈuːmd tʊ ɪnvˈɑːlv ðə tɹˈænsfɜːɹ əvə pɹˈoʊtɑːn ˈeɪtʃ plˈʌs fɹʌm ɐn ˈæsɪd tʊ ɐ bˈeɪs.