Extensive Definition
In typography, overstrike is a method of printing
characters that are missing from the printer's character
set. It was widely used around early 1990s. The character was
created by placing one character on the another one — for example,
overstriking "L" with "-" resulted in printing a "Ł"
character.
Many font renderers in computer programs invent
missing bold characters by overstriking the normal character with
itself, slightly horizontally offset. The horizontal offset is
essential, since unlike a typewriter, where repeating a letter in
exactly the same space will make it darker, most modern printers
will not darken repeated "strikes" to the same space. Actual bold
fonts are designed with some features thicker and others the same
size as a regular font, so the use of this "fake bold" is
considered undesirable from a typographic point of view; some high
end typesetting programs go so far as to refuse to fake bold in
this way.