A persistent security assault by someone trying to gain restricted access by altering data. There are multiple techniques, decryption for example, which can be used to lead the attack.

Active attack is an assault on system security that derives from an intelligent threat, i.e., an intelligent act that is a deliberate attempt (especially in the sense of a method or technique) to evade security services and violate the security policy of a system Source

Any attack that involves actions that are detectable as an attack by the target. A port scan is active because it can be detected by the remote host. Of course it isn't really an attack. An activ [..]

An active attack, in computing security, is an attack characterized by the attacker attempting to break into the system. During an active attack, the intruder will introduce data into the system as we [..]