see also in programming string (C) an alphabet \Sigma is a finite set a finite-sequence of elements in \Sigma is called a string the set of all strings in \Sigma is called \Sigma^{*}, which includes the empty string for a particular string x, the length of it is |x| the string of length zero is called \varepsilon a language is a subset of \Sigma^{*}, meaning its a set of strings Omer seems to call strings “words” sometimes. languages are boolean function over strings For every language L over \Sigma corresponds to a unique function f: \Sigma^{*} \to \{0,1\}, whereby if f(x) = 1, then x \in L; otherwise, if f(x) = 0, x \not \in L.