An acid is a molecule or ion capable of either donating a proton (i.e. hydrogen ion, H+), known as a Brønsted–Lowry acid, or forming a covalent bond with an electron pair, known as a Lewis acid.
ɐn ˈæsɪd ɪz ɐ mˈɑːlɪkjˌuːl ɔːɹ ˈaɪɑːn kˈeɪpəbəl ʌv ˈiːðɚ doʊnˈeɪɾɪŋ ɐ pɹˈoʊtɑːn ˈaɪ.ˈiː. hˈaɪdɹədʒən ˈaɪɑːn, ˈeɪtʃ plˈʌs, nˈoʊn æz ɐ bɹˈɑːnstᵻd lˈoʊɹi ˈæsɪd, ɔːɹ fˈɔːɹmɪŋ ɐ kˈoʊvələnt bˈɑːnd wɪð ɐn ᵻlˈɛktɹɑːn pˈɛɹ, nˈoʊn æz ɐ lˈuːɪs ˈæsɪd.