the water (e.g., the sea) that connects with the sky; the sky is like a house, and the water is like walls; ilhuicaatl is a synonym of teoatl (see attestations)
"teuatl: injc mjtoa teuatl, camo teutl, çan qujtoznequj maviztic vei tlamaujçolli, ioan itoca ilhujcaatl...çan iuhqujnma calli, noviiampa tlacçaticac: auh itech acitoc in atl, iuhqujnma acaltechtli, itech motlatzoa...." [teoatl. inic mitoa teoatl, camo teotl, zan quitoznequi mahuiztic huei tlamahuizolli, ihuan itoca ilhuicaatl...zan iuhquinma calli, nohuiyampa tlaczaticac: auh itech acitoc in atl, iuhquinma acaltechtli, itech motlatzoa...." (SW) = "teuatl. It is called teuatl [sea], not that it is a god; it only means wonderful, a great marvel. And its name is ilhuicaatl...just like a house; it stood resting in every direction, and it extended reaching to the water. It was as if the water walls were joined to it...." (1570s, central Mexico)