The Kleene star of a formal language 
is 
, where
consists of concatenations
of 
words from 
and 
contains only the empty word of
length 0. Thus 
includes the empty word, irrespective of whether 
does. It contains finite concatenations
rather than infinite words.
Kleene star is an operation in regular expressions. For a regular language, the minimum possible maximum nesting depth of Kleene stars among regular expressions denoting the language defines its star height; when complement is also allowed, the corresponding minimum depth is generalized star height (Kirsten 2005, p. 492).
