Creed, belief, formula or confession of religious belief.
The belief system that is based upon a firm conviction in all the fundamentals of faith and of the Oneness of Allah, i.e.
It is derived from the verb aqada meaning to firmly bind and knot. Whatsoever settles in the heart in a firm and definitive manner is said to be that person's aqeedah.
In the technical sense it refers to the definitive faith and certain ruling that is not open to any doubt. It is called aqeedah because the person binds his heart upon it.