The word cryptex is a neologism coined by the author Dan Brown for his 2003 novel The Da Vinci Code, denoting a portable vault used to hide secret messages. It is a combination of the words cryptology and codex; "an apt title for this device" since it uses "the science of cryptology to protect information written on the contained scroll or codex" (p. 199 of the novel).
In the main part of Brown's novel, the characters (while pursued by various sinister agencies) are trying to access the secret to the Holy Grail by figuring out the passwords that will open two different cryptexes, one hidden within the other to provide extra security. In the 2006 movie based on the novel, only one cryptex is vital to the plot (though another cryptex is briefly seen in a flashback scene with Sophie as a child). Its password is the Black Cryptex's password.