أخ
Akh
A brother in faith; the plurals are ikhwah and ikhwān.
What it means
Akh means a brother, and in Islamic usage it also names a fellow believer. The Quran says, “The believers are but brothers” (49:10), and the Prophet is reported to have said that none of you truly believes until he loves for his brother what he loves for himself (Sahih al-Bukhari and Sahih Muslim). The feminine form is ukht, sister. After the migration to Medina, the Prophet joined emigrants from Mecca and helpers from Medina as brothers in pairs, and the bond of faith was given the weight of kinship. Muslims greet one another as akhī, my brother, for this reason.
Often misunderstood as
Translated only as “brother”, it loses the bond of shared faith that Muslims extend to one another across lineage.
Term and Arabic as listed in Ismail al Faruqi, Toward Islamic English (International Institute of Islamic Thought, 1995), p. 23, listed as AKH. The definition is restated here.