Definition:String

Definition
Let $\mathcal A$ be an alphabet of symbols.

A string (in $\mathcal A$) is a sequence of symbols from $\mathcal A$.

There is no limit to the number of times a particular symbol may appear in a given string.

Finite String
A finite string is a string with a finite number of symbols in it.

Concatenation
We use the notation $S T$ to mean the string $S$ followed by the string $T$.

That is, $S T$ is $T$ concatenated with $S$.

Length of Concatenation
Clearly the length of $S T$ is given by:
 * $\left|{S T}\right| = \left|{S}\right| + \left|{T}\right|$

Also see

 * Substring
 * Initial part
 * Null string
 * Word

Variants
Some sources use the word string to mean a finite string, i.e. what we have chosen to call a word.