E9 is another name for the infinite-dimensional affine Lie algebra  (also as E8+ or E8(1) as a (one-node) extended E8) (or E8 lattice) corresponding to the Lie algebra of type E8.
ˈiː nˈaɪn ɪz ɐnˈʌðɚ nˈeɪm fɚðɪ ˈɪnfɪnətdᵻmˈɛnʃənəl ɐfˈaɪn lˈaɪ ˈældʒᵻbɹə ˈɔːlsoʊ æz ˈiː ˈeɪt plˈʌs ɔːɹ ˈiː ˈeɪt wˈʌn æz ɐ wˈʌnnˈoʊd ɛkstˈɛndᵻd ˈiː ˈeɪt ɔːɹ ˈiː ˈeɪt lˈæɾɪs kˌɔːɹɪspˈɑːndɪŋ tə ðə lˈaɪ ˈældʒᵻbɹə ʌv tˈaɪp ˈiː ˈeɪt.