• Set of string of finite or infinte string numbers.
• L ={w1, w2, w3, w4} where L is language which can be english or any language.

• Any one sequence of [Input alphabet](#Input alphabet). or
• Any finite combination or finite sequence over input alphabet.

• There is no precise definition of input alphabet, so it can be anything which is usually a set. Generally denoted with $\Sigma$ (sigma) $\Sigma$ = {a, b} — string can be formed through taking any value inside the $\Sigma$ any number of times.
• eg. a, b . length of this string is length or cardinality ( |a| = 1, |b| =2)

$\Sigma$ = {0, 1, +, -} all these symbols in input alphabets are allowed. here, +0 and -0 are Different things.

1. Length
   • Numbers of symbols in a string. let W= a, W= |a| = 1.