In typography, the ease with which typeset copy is read. Related to legibility is readbility, which describes how clearly what is typeset has been written and can be understood. Copy can be readable but not legible, legible but not readable, both legible and readable (the ideal situation), or neither legible nor readable (the worst situation).
Legibility is related to the way we read. The human eye fixates on something each quarter of a second and takes in a group of words. It then jumps to the next fixation, etc. Each of these fixations is called a saccad, and saccadic jumps move the eye from one saccad point to another. Speed-reading approaches usually try to train the reader's eyes to make larger jumps and take in more words at one time.
Legibility has been reflected in the design of letterforms. Large x-height serif faces with a bolder print to them tend to score highly in legibility research. Additionally, it has been found that word spacing should be the width of a lowercase "i" and the leading should be slightly larger than the word spacing. Legibility research also has found that narrower line lengths, consistent word spacing, upper- and lowercase lettering (as opposed to all caps), and well-designed typefaces aid in efficient reading.