A replay attack is a type of cyber-attack where an intruder intercepts and records legitimate data transmission, such as passwords or credit card information. The attacker then uses this recorded data to impersonate the user and gain unauthorized access to a system or network. This type of attack is particularly dangerous because it can be used to bypass even strong security measures like encryption and firewalls. Replay attacks are commonly carried out using packet sniffers, which intercept and record data packets as they travel over a network.