The Kleene star of a formal language 
is 
,
where 
consists of concatenations
of 
words from 
and 
contains only the empty word. 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. Its maximum nesting depth defines star height; when complement is also allowed, the corresponding minimum depth is generalized star height.
