DEK - Data Encryption Key The key used to encrypt the data

e.g. Key: 1234 with AES 128 as encryption algorithem - 1234 is the DEK

KEK - Key Encryption Key

e.g. Encrypt (from DEK above) 1234 with 9999; 9999 is the KEK

Master Key or MEK - Master Encryption Key

This key is used to encrypt/decrypt DEK and KEK in transit; usually used for KEK not for DEK.