Monospace
A typeface where every glyph takes the same width, so columns line up.
See it in ASCII art. Try the sample: every character sits in a cell of the same width.
In plain words
In most typefaces an i is narrow and an m is wide. In a monospace typeface every character gets the same width. That keeps columns straight, which is why code is usually shown in monospace. ASCII art needs it too, because each character stands in for one block of the picture. Typewriters and old terminals worked this way.