Source : Free On-Line Dictionary of Computing

horizontal tabulation
     
         (tab, Control-I, HT, {ASCII 9}) A character which
        when displayed or printed causes the following character to be
        placed at the next "tabstop" - the column whose number is a
        multiple of the current tab width.  Commonly (especially in
        Unix(?)) the tab width is eight, so, counting from the left
        margin (column zero), the tab stops are at columns 8, 16, 24,
        up to the width of the screen or page.
     
        A tab width of four or two is often preferred when indenting
        program {source code} to conserve indentation.
     
        Represented as "\t" in {C}, {Unix}, and derivatives.
     
        (1999-07-05)