TrueCrypt is a freeware utility for OTFE (on-the-fly encryption). It creates a virtually encrypted disk within a file, encrypts a disk partition, or even the entire storage device. Its development was discontinued on May 28, 2014, but its source code is still available for compilation and use.
It was originally released in 2004 based on the software E4M (encryption for the masses). It runs on Microsoft Windows, Apple macOS, and GNU/Linux, and supports the AES (Advanced Encryption Standard), Serpent, and TwoFish encryption algorithms. It includes a feature called Plausible Deniability that creates a hidden "virtual volume" within another volume. On Microsoft Windows, it can create and run a hidden, encrypted operating system whose existence can be plausibly denied.