Lexical Meaning:

According to Abdullah Al Athari (as referenced by the Islamic Online University),

“The word ‘aqidah (belief) is derived from the word ‘aqd, which means tying, confirming, consolidation, affirmation, holding together, and making firm. It refers to certainty (yaqin) and being sure” (pg. 41)

Furthermore, commenting on Islamic ‘aqidah, he writes:

“Islamic ‘aqidah means certain belief in the Lordship and Divinity of Allah, in His names and attributes, in His angels, His books, His Messengers, the Last Day, the Divine decree both good and bad, and all the matters of the unseen and the basic principles that have been proven (in Islam), that on which there was consensus among the righteous salaf, complete submission to the commands and rulings of Allah, obedience to Him and following his Messenger (saw).” (pg. 42)

References:

Athari, Abdullah. Islamic Beliefs. Retrieved from: Islamic Online University