However, definitions such as semantic security are too strong to achieve with certain specialized encryption schemes.
Entropic security is a weaker definition that can be used in the special case where an attacker has very little information about the messages being encrypted.
Note that in practice entropically-secure encryption algorithms are only "secure" provided that the message distribution possesses high entropy from any reasonable adversary's perspective.
For these schemes, stronger definitions (such as semantic security or indistinguishability under adaptive chosen ciphertext attack) are appropriate.
Russell and Wang formalized a definition of entropic security for encryption.