Ignorable Characters

A Default Ignorable Character is not rendered by default, but may have an effect on the text. An example of an ignorable character is the soft hyphen (). The appearance of " therapist" is simply "therapist"; no visible glyph. However, the soft hyphen may be displayed as a hyphen at the end of a line.

 

Source: http://www.unicode.org/L2/L2002/02368-default-ignorable.html