aka symbol code

In a variable-length code, one assigns a codeword to each letter in an alphabet. Formally, a variable-length code is a function $C: \mathcal{X} \rightarrow A^*$, where $\mathcal{X}$ is the source alphabet, and $A$ is the code alphabet, and $\cdot^*$ is the Kleene star.

The extension of $C$ is the natural expension of $C$ to $\mathcal{X} ^*$

The codewords are all the elements of the codomain of $C$.

$C$ is uniquely decodable if $C^*$ is one-to-one.

Prefix code

(IC 2.2) Symbol codes - terminology and notation

An example is Morse code

Non-commutative Symbolic Coding