Alphabet

An arbitrary (usually finite) set of elements denoted by the symbol $\mathrm{\Sigma }$

Each element of the alphabet (in concurrency) is an "action" that does changes.

Examples:
$\mathrm{\Sigma }=\{0,1\}$
$\mathrm{\Sigma }=\{0,1\}$
$\mathrm{\Sigma }=\{0,1,\dots ,9\}$

See also: Sequence

Sigma Star
${\mathrm{\Sigma }}^{\ast }=a_1\dots a_k\mid a_i\in \mathrm{\Sigma }\wedge k\ge 0$