Known-plaintext attack

The known-plaintext attack (KPA) is an attack model for cryptanalysis where the attacker has samples of both the plaintext and its encrypted version (known as ciphertext version) then they can use them to expose further secret information after calculating the secret key. 
Encrypted file archives such as ZIP are very prone to this attack. For example, an attacker with an encrypted ZIP file needs only one unencrypted file from the archive which forms the "known-plaintext". Then using some publicly available software they can instantly calculate the key required to decrypt the entire archive.
Classical ciphers are typically susceptible to known-plaintext attack. For example, a Caesar cipher can be solved using a single letter of corresponding plaintext and ciphertext to decrypt entirely.