Quantum Encryption is a two-party functionality that allows the establishment of a secure quantum channel, capable of transmitting quantum (or quantumly-encoded) messages, where no information from the message is leaked to the adversary who is present on the channel. These schemes are often considered against unbounded quantum adversaries that are present on the channel.  Quantum encryption schemes are often realised by symmetric-key settings where they often consist of an encoding and a decoding algorithm, and a pre-shared secret key. One of the most famous information-theoretic secure protocols known for this functionality is Quantum One Time Pad (QOTP) [1]

Classical one-time pad

Secure transmission of quantum information.
Quantum Encryption Schemes are often useful as subroutines in other quantum protocols and functionalities, but can also have real-world applications as quantum-secure schemes. 

The main property of a quantum encryption scheme is security against a quantum adversary, which can be defined as no quantum adversary should be able to extract any information about the quantum message from the quantum ciphertext. The security can be formalised with different cryptographic definitions, such as information-based definitions or “semantic security” notions [].

Additionally, a quantum encryption can have these extra optional properties:

• Non-malleability: it ensures that adversaries cannot manipulate the plaintext by acting on the ciphertext [2]
• Authentication: An encryption can be additionally authenticated, meaning that it is guaranteed that the ciphertext is coming from a genuine party. In the quantum world, authenticated quantum encryption and Non-malleable encryption are tightly linked, unlike in the classical world. 
• Composability: An encryption scheme is composable if the security has been proved within a composable framework, which in turn allows for the scheme to be combined and used within other protocols without losing the security guarantees. 

[1] Mosca, Michele, Alain Tapp, and Ronald de Wolf. “Private quantum channels and the cost of randomizing quantum information.” arXiv preprint quant-ph/0003101 (2000).

[2] Alagic, Gorjan, and Christian Majenz. “Quantum non-malleability and authentication.” In Annual International Cryptology Conference, pp. 310-341. Cham: Springer International Publishing, 2017.