Heartbleed - Definition from the Hotspot Shield Glossary
Heartbleed is a serious security bug that is concerned with the OpenSSL library (OpenSSL is a popular implementation of the TLS protocol). The bug derives from input errors and validation when implementing TLS, and particularly the heartbeat extension.
The bug can be exploited by hackers who wish to obtain user data; the attacker will send a malformed request that will unwittingly cause the user to provide the attacked with data that is held in their memory cache. The bug was found by various businesses including Google and was reported to the OpenSSL developers - It was reported that a huge number of TLS-enabled websites were vulnerable to the bug and over 30,000 X.509 digital certificates could have been compromised.