What is Trigraph chararcter?

Answer 1

Trigraph Character are used when your keyboard does not support some special character: Following are few Trigraph character.

??           -->            #
??(          -->            [
??)          -->            ]
??<          -->            {
??>          -->            }
??!          -->            |
??/          -->            \
??'          -->            ^
??-          -->            ~

Answer 2

A trigraph is a three-character replacement for a special character. A trigraph can be used in place of a symbol that is not present on a keyboard or in a character set.
(Note: Some programs cannot properly interpret trigraph)

Trigraph Character

??           -->            #
??(          -->            [
??)          -->            ]
??<          -->            {
??>          -->            }
??!          -->            |
??/          -->            \
??'          -->            ^
??-          -->            ~

However trigraph is not just limited to the computer. In a language a trigraph is a set of three consecutive letters that represents a single sound. For example, in the term "bureau of investigation" the letters e, a, and u together represent a long "o" sound, as in "oh."