- plaintext     = The secret text (message, password, ...) you want to encrypt.
- ciphertext    = A plaintext that has been encrypted.
- cipher        = A mathematical function you provide a plaintext, key and sometimes that produces a ciphertext. For example: AES.
- hash function = A mathematical function that produces a hash which cannot be reversed back to plaintext.
- key           = A secret key that allows someone to decrypt a ciphertext back into a plaintext.
- secret        = see key.
- PBKDF2 = password based key derivation function (number 2 I guess). Used for hashing passwords.