An Arrhenius acid is a substance that, when added to water, increases the concentration of H+ ions in the water.
ɐn ɚrhˈiːnɪəs ˈæsɪd ɪz ɐ sˈʌbstəns ðˈæt, wˌɛn ˈædᵻd tə wˈɔːɾɚ, ˈɪŋkɹiːsᵻz ðə kˌɑːnsəntɹˈeɪʃən ʌv ˈeɪtʃ plˈʌs ˈaɪɑːnz ɪnðə wˈɔːɾɚ.