Iʿtikāf means residing in a specific place. In the sharia (the sacred law), it signifies residing in a particular place with a specific act and a particular ḥāl (spiritual state), with the aim of drawing near to Allah. This form of iʿtikāf is religiously recommended (mandūb). Fulfilling a vowed iʿtikāf is obligatory (wājib). It should be performed in a manner befitting Allah, by preferring Him (by making Him the goal), and with the intention of residing with Him. To reside with Allah is better than to reside with one's nafs (the lower self).
As for the acts specific to iʿtikāf, there are different opinions on this matter. Some hold that no acts other than prayer, dhikr (remembrance of God), and recitation of the Qur'an should be performed. Others say that any good deed pertaining to the Hereafter may be performed. In my view, any good deed that does not require leaving the place of residence may be done. If one leaves the place of residence, it does not count as iʿtikāf. In my opinion, no other condition is necessary. (Futūḥāt, I, 791)