Principal English Translation: 

mountain lion, cougar, or a wild cat; also, a name given to a child
Fray Bernardino de Sahagún, Primeros Memoriales, ed. Thelma D. Sullivan (Norman: University of Oklahoma Press, 1997), 254.

Orthographic Variants: 
mimistin, mimiztin
Alonso de Molina: 

miztli. leon.
Alonso de Molina, Vocabulario en lengua castellana y mexicana y mexicana y castellana, 1571, part 2, Nahuatl to Spanish, f. 57v. col. 1. Thanks to Joe Campbell for providing the transcription.

Frances Karttunen: 

MIZ-TLI pl: MĪMIZTIN feline, mountain lion / león (M) The diminutive of this,
Frances Karttunen, An Analytical Dictionary of Nahuatl (Norman: University of Oklahoma Press, 1992), 149. MIZTŌN is used for the domestic cat.

Attestations from sources in English: 

mimniztin = mountain lions (plural)
Antonio Rincón, Arte mexicana: Vocbulario breve, que solamente contiene todas las dicciones ue en esta arte se traen por exemplos (1595), 5v.

In the ongoing discussion of cat terms Kier Salmon brings up the subject of Nahuatl *ocelotl*. The latter (*o:ce:lo:tl*) is actually 'jaguar' par excellence, whereas *tla'coo:ce:lo:tl *(literally, 'semi-jaguar') is 'ocelot'. I have no idea why Dibble and Anderson (or Anderson and Dibble) decided to continue translating plain *o:ce:lo:tl* simply as 'ocelot', which is quite a misleading definition. The zoologist they consulted, Stephen Durrant, recommended 'jaguar' over their 'ocelot' (Florentine Codex, Bk. 11, 1963, p. 1, fn. 2), but they stuck to their translation. As a result, they rendered both *o:ce:lo:tl* and *tla'coo:ce:lo:tl* as "ocelot".

The indigenous consultants for the Florentine Codex descriptions, however, clearly regarded the *tla'coo:ce:lo:tl*, which they also named the * tla'comiztli* ('semi-puma'), as a separate animal, not merely a different kind of *o:ce:lo:tl*, as the following passage (FC 11: 3) implies: , translated by Dibble and Anderson as "OCELOT Also they name it *tlacomiztli*. It is small, squat, rather long, the same as a Castilian cat; ashen, whitish, varicolored -- varicolored like an ocelot, blotched with black". The significant translational oddity here is the comparison: "OCELOT [...] small, [...], the same as a Castilian cat, [...] -- varicolored like an *ocelot*". Clearly, 'OCELOT [...] -- varicolored like a *jaguar*' would be a better fit.

We don't know whether Aztec *o'o:ce:lo'* only wore jaguar skins, or whether some of them were running around in ocelot uniforms. The evidence strongly favours a primary association with jaguars, for cultic, cosmological and ideological reasons. Thus, for practical purposes, the translation "jaguar warriors" is still okay.
Gordon Whittaker, Aztlan Listserv posting, Feb. 25, 2012.

quitzonaya yn teponaztlí- Yhuá cololí; cuauhcoyolí, yhuá hoccequine Yaotlatquítl yca miec tzatziliztli, tla huelecayo tíca cayȗh, Ozelomê mimiztín = quitzonaya in tepona:ztli i:hua:n coyoli, cuauhcoyolin, i:hua:n oc cequi:ne ya:o:tlatquitl i:ca miec tzahtziliztli, tlahue:lehcayo:tica ca iuh o:celo: meh, mimi:ztin = they beat the lateral log drum and trumpets, wooden bells, along with other war property, with much furious shouting, like coyotes, jaguars, and mountain lions.
Anónimo mexicano, ed. Richley H. Crapo and Bonnie Glass-Coffin (Logan, UT: Utah State University Press, 2005), 39.

yn alcalde mayor quinechicozque mimistin = the alcalde mayor ordered that cats be rounded up
Here in This Year: Seventeenth-Century Nahuatl Annals of the Tlaxcala-Puebla Valley, ed. and transl. Camilla Townsend, with an essay by James Lockhart (Stanford: Stanford University Press, 2010), 134–135.

yuhqui ce ichcaconetzintli, inic immac mohuetzititiuh in ocelome, in quautla mimiztin = as if he were a little lamb, as he went to fall into the hands of the jaguars, the forest pumas (early sixteenth century, Central Mexico)
Louise M. Burkhart, Before Guadalupe: The Virgin Mary in Early Colonial Nahuatl Literature, Institute for Mesoamerican Studies Monograph 13 (Albany: University at Albany, 2001), 94.

mizehualtilmahtli = mountain lion or puma skin cape
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), 194.

Attestations from sources in Spanish: 

Inimequez nahualtin tlanonotzalo quemen mocuepa quemanian de yolcatl, de chichi, de miztli, de cuanaca, o nozo mocuepa in texcaltin. = Cuentan que estos nahuales a veces se vuelven animales--perros, gatos, gallinas--o se vuelven peñascos. (s. XX, Milpa Alta)
Los cuentos en náhuatl de Doña Luz Jiménez, recop. Fernando Horcasitas y Sarah O. de Ford (México: UNAM, 1979), 32–33.