First: I'tikaf linguistically means to stay on something and adhere to it, and to confine oneself to it. This is derived from His saying, Glory be to Him: (What are these statues to which you are devoted?) Al-Anbiya: 52, and His saying, Glory be to Him: (They are devoted to idols of theirs) Al-A'raf: 138. I'tikaf means confinement and stopping, as He, Glory be to Him, said: (And the sacrificial animals are to be brought to the place of sacrifice) Al-Fath: 25. See: Talabat al-Talabah p. 26, and Al-Maghrib p. 324. Secondly: I'tikaf terminologically refers to a fasting person staying in a mosque of congregation with the intention. He, Glory be to Him, said: (And purify My House for those who perform Tawaf and those who are devoted) Al-Baqarah: 125. The linguistic meaning is present with an additional description. A mosque of congregation is one that has an imam and a muezzin, and in which the five daily prayers are performed, or may not be performed. Therefore, I'tikaf is valid in a congregational mosque where Friday prayers are held, even if not all prayers are performed there. See: Al-Wiqayah p. 244, and Tabyin al-Haqaiq 1: 347, and Al-Talimat al-Mardiyyah p. 183, and Al-Hadiyyah al-Alaiyyah p. 183.