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Generally, any way of representing something using numbers, letters and symbols. For instance:
- codes are used for security; we turn words and sentences into strings of numbers, letters and symbols, which are hard to decode.
- codes are used in computer programming; instructions which humans can read are turned into code (numbers, letters and symbols) which a computer can read.
- a bar code represents information, often the name and price of a product, using a series of vertical lines.

In information theory: A map f : I → J^* where I is a source alphabet, and J^* is the set of all finite strings from an encoding alphabet J , typically J = { 0,1 } . A codeword is an image of some u ∈ I .
A code is said to be decipherable if its extension to the set of all strings of source letters given by the obvious concatenation

f(u1u2 …un)=f(u1)f(u2) …f(un)

is injective. See also prefix-free, Kraft inequality.