The Arabic word illa is a negative word corresponding to the English except, only and but[1].
Use
It is an often recurring phrase in the Qur'an, often used to give strength to statements by first negating all possibilities, and then referring to a subject. For example, in the Islamic Creed (Arabic: Shahadah):
This can also be seen in the prayer La hawla wa la quwwata illa billah, There is neither change nor power except by means of God.
References
Illa (Arabic) Wikipedia(Text) CC BY-SA