Principal English Translation:
a small terrace projecting out from the foot of the pyramid stairway
Fr. Bernardino de Sahagún, Florentine Codex: General History of the Things of New Spain; Book 2 -- The Ceremonies, no. 14, Part III, eds. and transl. Arthur J. O. Anderson and Charles E. Dibble (Santa Fe and Salt Lake City: School of American Research and the University of Utah, 1951), 47.
Attestations from sources in English:
This was where the bodies of the sacrificed captives landed after they rolled down the staircase of the pyramid of Huitzilopochtli (after having their hearts cut out):
ynjc oalaci apetlac = thus they reached the terrace at the base of the pyramid
Fr. Bernardino de Sahagún, Florentine Codex: General History of the Things of New Spain; Book 2 -- The Ceremonies, no. 14, Part III, eds. and transl. Arthur J. O. Anderson and Charles E. Dibble (Santa Fe and Salt Lake City: School of American Research and the University of Utah, 1951), 47.