The name is of uncertain origin. The historic medieval names were Lewe, Lewa (the same historic name as Levice), Lewa de cidca fluviom Vag, later Ilava. The form Illava is known from the 19th century and was used after the Austro-Hungarian Compromise of 1867.
It is located in the Ilava Basin near the Váh river at the foothills of Strážovské vrchy mountains, near the cities of Považská Bystrica and Trenčín. In addition to the main settlement, it also has “boroughs” of Iliavka and Klobušice (both since 1969, latter with a Classic castle).
The first written reference to the town dates from 1332/1337, the town charter stems from 1339. The settlement developed below a Gothic castle (bearing the same name) from the 13th century, turned into a Renaissance fortification in the 16th century, into a monastery in 1693 and finally, in 1856, into a prison. A concentration camp was in the prison in 1938.