A side-channel attack is a special type of attack against a cryptosystem.
The attacker does not try to exploit weaknesses in the mathematical basis of the underlying cryptosystem - instead he is making use of a so-called side-channel.
Unintended behaviour in terms of the reaction of a cryptographic system is called a side-channel. Both the wrong specification of a cryptographic system and the incorrect implementation can lead to a side-channel. In both cases an attacker can gather internal information about the cryptographic system by provoking certain reactions of the targeted system.
Further information about side-channel attacks can be found in the script.