In Spain. Portugal and in some countries South America frequently used name component or synonym for many different white grape varieties. The confusion is reinforced by different spellings ("o" and "u"). Especially with the cultivars grown in Portugal and Spain there is often uncertainty, which exactly meant. Also the Portuguese DOC or Spanish DO regulations as well as grape variety catalogs are no help, if only "Torrontés" is indicated there.
Source: Wine Grapes / J. Robinson, J. Harding, J. Vouillamoz / Penguin Books Ltd. 2012