Legibility is the degree to which glyph

Glyph

A glyph is an element of writing: an individual mark on a written medium that contributes to the meaning of what is written. A glyph is made up of one or more graphemes....

s (individual characters) in text

Written language

A written language is the representation of a language by means of a writing system. Written language is an invention in that it must be taught to children, who will instinctively learn or create spoken or gestural languages....

 are understandable or recognizable based on appearance. "The legibility of a typeface is related to the characteristics inherent in its design ... which relate to the ability to distinguish one letter from the other." Legibility includes factors such as "x-height

X-height

In typography, the x-height or corpus size refers to the distance between the baseline and the mean line in a typeface. Typically, this is the height of the letter x in the font , as well as the u, v, w, and z...

, character shapes, stroke contrast, the size of its counters, serifs or lack thereof, and weight."

Legibility is different from readability

Readability

Readability is the ease in which text can be read and understood. Various factors to measure readability have been used, such as "speed of perception," "perceptibility at a distance," "perceptibility in peripheral vision," "visibility," "the reflex blink technique," "rate of work" , "eye...

which refers to entire words, sentences, and paragraphs.
Readability is influenced by line length, primary and secondary leading, justification, typestyle, kerning, tracking, point size, etc.

In practical application some attributes cross over and affect both.