Principal English Translation: 

a sharp-bladed instrument of obsidian; also, Itztli ("Obsidian Blade") was a deity that was part of the Tezcatlipoca Complex of deities that relate to power, omnipotence, often malevolence, feasting and revelry.
"Table 3. Major Deities of the Late Pre-Hispanic Central Mexican Nahua-Speaking Communities." Handbook of Middle American Indians, Volume 6: Social Anthropology, ed Manning Nash (Austin: University of Texas Press, 1967).

Orthographic Variants: 
Frances Karttunen: 

ĪTZ-TLI obsidian / obsidiana o fragmentos de obsidiana utilizados como cuchillos, navajas de afeitar, lancetas, flechas, espejos, etc. (S) [(1)Tp.224]. See ĪTZTIC.
Frances Karttunen, An Analytical Dictionary of Nahuatl (Norman: University of Oklahoma Press, 1992), 109.

Lockhart’s Nahuatl as Written: 

ītztli. obsidian, sharp-bladed instrument of obsidian; it is not entirely certain that the i of the root is long.
James Lockhart, Nahuatl as Written: Lessons in Older Written Nahuatl, with Copious Examples and Texts (Stanford: Stanford University Press and UCLA Latin American Studies, 2001), 221.

Attestations from sources in English: 

itzcopeuhquen in itequiuh centomin = The obsidian-blade makers' tax is 1 tomín (Coyoacan, mid-sixteenth century)
Beyond the Codices, eds. Arthur J.O. Anderson, Frances Berdan, and James Lockhart (Los Angeles: UCLA Latin American Center, 1976), Doc. 25, 144–145.

cenca vel quiximati in tecpatl, in itztli = they are very well acquainted with flint and obsidian (Tlatelolco 1540–80)
James Lockhart, Nahuatl as Written: Lessons in Older Written Nahuatl, with Copious Examples and Texts (Stanford: Stanford University Press and UCLA Latin American Studies, 2001), 195.

itznepaniuhqui tilemahtli = "The cape with the crossed obsidian knives design"; Seler translated it as "the cape with crossed obsidian points (or obsidian points at the intersections)"
Justyna Olko, Turquoise Diadems and Staffs of Office: Elite Costume and Insignia of Power in Aztec and Early Colonial Mexico (Warsaw: Polish Society for Latin American Studies and Centre for Studies on the Classical Tradition, University of Warsaw, 2005), 193.

Obsidian knives (ytztli) are given as some of the essential items found in the "devil's houses" (Sahagún).
Fray Bernardino de Sahagún, Primeros Memoriales, ed. Thelma D. Sullivan, et al. (Norman: University of Oklahoma Press, 1997), 116.

intla omjc piltontli, itztli qujcalaquja in ijtic cioatzintli in ticitl = if the baby had died, the midwife inserted an obsidian knife within the woman (central Mexico, sixteenth century)
Fr. Bernardino de Sahagún, Florentine Codex: General History of the Things of New Spain; Book 6 -- Rhetoric and Moral Philosophy, No. 14, Part 7, 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, 1961), 157.

"Itztli, the obsidian knife, was one form of the terrible goddess Itzpapalotl, Obsidian Knife Butterfly, and several of the Aztec goddesses wore knives as skirts or carried them on their backs like babies. Sacrificial knives are often shown with faces, or even as animated gods chasing and devouring victims."
John C. Whittaker, Flintknapping: Making and Understanding Stone Tools (2010).

"...a stone knife, and in this manifestation he was known as Itztli, the knife-god. He is thus brought into intimate relation with the hunting deities, of whom this weapon was a symbol."
Thomas A. Joyce, Mexican Archaeology (2012), 45.

Attestations from sources in Spanish: 

Itztli, nauaja.
Luis González Obregón, Colección de gramáticas de la lengua mexicana (1904), 19.