Tawheed

التَّوْحِيدُ وَهُوَ إِفْرَادُ اللهِ بِالْعِبَادَةِ

Tawheed is the singling out of Allaah in [all] worship.

(From Al Usool Ath Thalaathah by Muhammad ibn Abdil Wahhaab)

For a longer explanation in pdf format CLICK HERE