Wikipedia
Crypto-Judaism is the secret adherence to Judaism while publicly professing to be of another faith; practitioners are referred to as "crypto-Jews" (origin from Greek kryptos - κρυπτός, 'hidden'). The term crypto-Jew is also used to describe descendants of Jews who maintain some Jewish traditions of their ancestors while publicly adhering to other faiths.
The term is especially applied historically to European Jews who—outwardly or forcedly—professed Catholicism, who were also known as Anusim or Marrano. The phenomenon is especially associated with renaissance Spain, following the June 6, 1391, Anti-Jewish pogroms and the expulsion of the Jews in 1492 unless they converted. Later under its Blood Purity Laws, Spain restricted explorers and settlers in the New World to "Old Christians" of three generations or more.