yolloxochitl.

Headword: 
yolloxochitl.
Principal English Translation: 

a type of magnolia (magnolia glauca), a heart-shaped flower (often associated with the Virgin Mary, for its purity and beauty)
Louise M. Burkhart, Before Guadalupe: The Virgin Mary in Early Colonial Nahuatl Literature, Institute for Mesoamerican Studies Monograph 13 (Albany: University at Albany, 2001), 20.

Orthographic Variants: 
yoloxochitl
IPAspelling: 
yoːllohʃoːtʃitɬ
Alonso de Molina: 

yolloxochitl. flor muy olorosa de hechura de corazon.
Alonso de Molina, Vocabulario en lengua castellana y mexicana y mexicana y castellana, 1571, part 2, Nahuatl to Spanish, f. 41r. col. 1. Thanks to Joe Campbell for providing the transcription.

Frances Karttunen: 

YŌLLOHXŌCHI-TL magnolia / flor muy olorosa de hechura de corazón (M), flor de corazón (K)[(1)Cf.76r]. See YŌLLOH-TLI, XŌCHI-TL.
Frances Karttunen, An Analytical Dictionary of Nahuatl (Norman: University of Oklahoma Press, 1992), 342.

Attestations from sources in English: 

yolloxochitl ‘heart flower,’ or Talauma mexicana Don, a white flower with numerous medicinal and ritual uses (late 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), 120.

"a tall tree with leaves like the citron, but twice as large and umbilical; the flowers are heart-shaped...and if they are mixed with cacao shells or with a draft made from cacao, they strengthen the heart and the stomach" (Central Mexico, 1571–1615)
The Mexican Treasury: The Writings of Dr. Francisco Hernández, ed. Simon Varey, transl. Rafael Chabrán, Cynthia L. Chamberlin, and Simon Varey (Stanford: Stanford University Press, 2000), 119.

Variations include tlacayolloxochitl (large, beautiful rose-like flowers) and izcuinyolloxochitl (magnolia de los perros, used to cure epilepsy in dogs, and other medicinal purposes).

Attestations from sources in Spanish: 

The root is ground with ground flowers (tetzmixochitl, tlacoizquixochitl, yolloxochitl, eloxochitl, ocoxochitl, plus red earth, white earth, and eztetl, all mixed with water and taken as a beverage. On the exterior of the stomach, one would also apply a stone found in the river that appeared to have large pearls in it.
Martín de la Cruz, Libellus de medicinalibus indorum herbis; manuscrito azteca de 1552; según traducción latina de Juan Badiano; versión española con estudios comentarios por diversos autores (Mexico: Fondo de Cultural Económica; Instituto Mexicano del Seguro Social, 1991), 51 [34 v.].

yolloxochitl = una especia de magnolia; quiere decir flor parecida al corazón; no se podría traducir de ninguna manera flor del corazón, pues no significaría nada.
Rémi Siméon, Diccionario de la lengua náhuatl o mexicana (Mexico: Siglo XXI, 1988), xxxix.

En el Códice Mexicanus, debajo de la glosa "(I)ulius" para indicar el mes de julio, "se dibujó una flor, quizá la yolloxochitl, cuya primera parte (yollo/iollo) parece ser un acercamiento fonético al mes de julio o iulio en latín." (s. XVI, Ciudad de México)
Michel Oudijk agradece a Sebastián van Doesburg por la referencia al nombre de esta flor. Ver: Michel R. Oudijk, "El almanaque médico," en El Códice Mexicanus, tomo I, eds María Castañeda de la Paz y Michel R. Oudijk, eds. (México: UNAM, 2019), 29.

themes: 
Anatomy
Flowers
Health, Sickness, Wellness, Medicine
Plants, Trees, Flora
Religion (Christian)