Thoughout history conquerors have forced the vanquished to renounce the religion or faith of their heritage. In modern history Jews and Moslems have been especially singled out.  The secret adherence to Judaism while publicly professing to be of another faith; people are referred to as "Crypto-Jews". The term is also used to describe descendants of Jews who still (generally secretly) maintain some Jewish traditions, often while adhering to other faiths.
Anusim, plural for anús, means forced ones in Hebrew. This is a legal terminology applied to a Jew who has been forced to abandon Judaism against his or her will, and who does whatever is in his or her power to continue practicing Judaism under the forced condition. It derives from the Talmudic term aberá be'ones [TB: Abodá Zará 54a], meaning "a forced transgression". The term is essentially a question of status based on observance of Jewish Law.
Converso (Spanish and Portuguese for "a convert", from Latin conversus, "converted, turned around"), and its feminine form conversa, referred to Jews or Muslims or the descendants of Jews or Muslims who had converted to Catholicism in Spain and Portugal, particularly during the 14th century and 15th century. They were also called called New Christians. In the minds of many churchmen and Christians, conversos were still identified as Jews because they typically continued to live in Jewish communities and practice occupations associated with Jews.
Source: Association of Crypto Jews and other sites