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 during the Inquisition (i.e., the Moriscos and their usage of Aljamiado). Related to this term are the concepts of crypto-Muslim subgroups, such as crypto-Sunni Muslims and crypto-Shia Muslims. Each of these Muslim subgroups has suffered persecution at various times under various leaders where it was advantageous to appear as part of the dominant (sometimes also Muslim) group.
Taqiya is a sort of Crypto-Islam.
References
Crypto-Islam Wikipedia(Text) CC BY-SA