The full-disk encryption feature introduced in OS X Lion is FileVault 2.
Earlier versions of FileVault, beginning with Mac OS X Panther in 2003, encrypted only a user’s home directory inside an encrypted disk image. FileVault 2, introduced with OS X Lion in 2011, was a major redesign that encrypted the entire startup volume, including the operating system and user data.
FileVault 2 also changed the underlying encryption approach. It used XTS-AES encryption and allowed authorized users to unlock the drive during startup. A recovery key could be created in case the login password was forgotten, making the system practical for both individuals and organizations.
A common mix-up is treating FileVault and FileVault 2 as identical. “FileVault” is the general Apple technology name, but the Lion-era full-disk feature is specifically called FileVault 2. BitLocker is Microsoft’s comparable Windows feature, not Apple’s.