Praise be to Allaah.
Aqeedah refers to those matters which are believed in, with certainty and conviction, in
ones heart and soul. They are not tainted with any doubt or uncertainty.
The Arabic word aqeedah stems from the root aqada, which conveys meanings of
certainty, affirmation, confirmation, etc. In the Quraan, Allaah says (interpretation of
"Allaah will not punish you for what is unintentional in your oaths, but He will
punish you for your deliberate oaths (bimaa aqqadtum al-aymaan)
The verb paraphrased here as "deliberate oaths" is aqqada/taqeed, which refers to
when one has determination in the heart. It may be said in Arabic, aqadal-habl (the
rope was tied), i.e., it was pulled tight together. The word itiqaad (belief) is also
derived from this root, and has the meaning of tying up and making strong. The phrase
ataqadtu kadhaa (I believe such and such) means: I am convinced of it in my heart;
this is a rational conviction.
In Islam, aqeedah is the matter of knowledge. The Muslim must believe in his heart
and have faith and conviction, with no doubts or misgivings, because Allaah has told
him about aqeedah in His Book and via His Revelations to His Messenger
(peace and blessings of Allaah be upon him).
The principles of aqeedah are those which Allaah has commanded us to believe in, as
mentioned in the aayah (interpretation of the meaning):
"The Messenger believes in what has been sent down to him from his Lord, and (so
do) the believers. Each one believes in Allaah, His Angels, His Books and His
Messengers. They say, We make no distinction between one and another of His
Messengers and they say, We hear, and we obey. (We seek) Your forgiveness, our
Lord, and to You is the return (of all)." [al-Baqarah 2:285]
- and as defined by the Prophet
(peace and blessings of Allaah be upon him) in
the famous hadeeth which describes how Jibreel came to him and asked him about
Islam etc.: "Eemaan (faith) is to believe in Allaah, His angels, His Books, the meeting
with Him on the Last Day, His Messengers, and the Day of Resurrection."
So in Islam, aqeedah refers to the matters which are known from the Quraan and
sound ahaadeeth, and which the Muslim must believe in his heart, in acknowledgement
of the truth of Allaah and His Messenger.
(Source: Sharh Lam’ah al-I’tiqaad by Ibn al-Uthaymeen, and al-‘Aqeedah fi-Allaah, by ‘Umar al-Ashqar).