'Ibaadah

Imam al-Qurtubee (d.671H) (rahimahUllaah) said, 

"The root of 'ibaadah is humility and submissiveness. The various duties that have been prescribes upon the people are called 'ibaadat (acts of worship), since what is required is that these acts of worship must be done with humility and submissiveness to Allaah - the Most High."

[Al Jaami' li-Ahkaamil-Qur'aan (1/225) and (17/56) of al-Qurtubee]