在密码学中，密码（英语：cipher，或cypher），又称加密（encipherment），指的是用来加密或解密的算法。使用密码，可以将一段明文的内容，经过特定进程，转换成无法理解的密文；或是反向，将密文转换成可以被理解的明文。

在现代密码系统中，密码可以做出以下大略的区分：

In cryptography, a cipher (or cypher) is an algorithm for performing encryption or decryption—a series of well-defined steps that can be followed as a procedure. An alternative, less common term is encipherment. To encipher or encode is to convert information into cipher or code. In common parlance, 'cipher' is synonymous with 'code', as they are both a set of steps that encrypt a message; however, the concepts are distinct in cryptography, especially classical cryptography.