Crypto-Islam is the secret adherence to Islam while publicly professing to be of another faith; people who practice crypto-Islam are referred to as "crypto-Muslims." The word has mainly been used in reference to Spanish Muslims and Sicilian Muslims during the Inquisition (i.e., the Moriscos and Saraceni and their usage of Aljamiado).
Some historic examples include Ahmad ibn Qasim Al-Hajarī, 16th-century crypto-Muslim from Spain who authored a book recounting how he organized his escape from Spain to Morocco, and also including a refutation of the Catholic opinions about Jesus. The books also included details about how crypto-Muslims lived in Spain. He later became Ambassador of Morocco to Spain.
Sicilian crypto Muslims maintained large amounts of their traditions despite massive amounts of oppression and recently a small community of crypto Muslims on the Island have converted back to Islam following the opening of several mosques and Islamic centers  However some still practice their religious ceremonies as Catholics.
There are claims that Armah, who ruled the Kingdom of Aksum and gave refuge to early Muslim converts, was a crypto-Muslim.