Definition for legibility & readability

Legibility:
Legibility refers to the recognizability of individual glyphs (the individual markings that signify the semantic character(s)).
a measure of how easy it is to distinguish one letter from another in a given typeface.
“Legibility” is based on the ease with which one letter can be told from the other.
Get a reader’s attention and interest
Legibility is mostly a function of typeface design. It’s a measure of how easy it is to recognize one letter or word from another and how easy blocks of text are to read.
Legibility is a function of typeface design. It’s an informal measure of how easy it is to distinguish one letter from another in a particular typeface.
Legibility is primarily the concern of the typeface designer, to ensure that each individual character or glyph is unambiguous and distinguishable from all other characters in the font. Legibility is also in part the concern of the typographer to select a typeface with appropriate clarity of design for the intended use at the intended size. An example of a well-known design, Brush Script, contains a number of illegible letters since many of the characters can be easily misread especially if seen out of textual context.
A range of factors influence a glyph’s legibility:
stroke, width, angle (of the stroke), style (e.g. roman, full-capitals), slant (of the whole style), color (actual color and typographic color, i.e. contrast), background color, and more.
How legible a typeface is designed to be depends on its purpose.
Type size, Type weight, Type style/posture, Line length, Letterspacing, Wordspacing, Linespacing, Justified vs flush left/right, Lowercase/all caps/small caps, Contrast between type and background, Serif vs sans-serif

Readability:
Readability refers to the recognizability of whole words, sentences, paragraphs, tables, or whatever the text en masse constitutes.
Readability – how easy words, phrases, and blocks of text can be read. Readability describes how a typeface is used on the page.
“Readability” is the ease with which the eye can absorb the message and move along the line.
Be easy to read
Readability is a function of how typefaces are used. It’s about how inviting your type is to read and about getting the viewer to want to read it.
Readability applies to the overall reading experience. It’s macro-typography and it’s about making type aesthetically pleasing in order to make it more inviting to read.
Readability, on the other hand, is dependent upon how the typeface is used. Readability is about typography. It is a gauge of how easily words, phrases and blocks of copy can be read.
Readability is primarily the concern of the typographer or information designer. It is the intended result of the complete process of presentation of textual material in order to communicate meaning as unambiguously as possible. A reader should be assisted in navigating around the information with ease, by optimal inter-letter, inter-word and particularly inter-line spacing, coupled with appropriate line length and position on the page, careful editorial “chunking” and choice of the text architecture of titles, folios, and reference links.
A range of macro factors affect readability:
the measure (line length), the leading (line height or spacing), justification or alignment, the style of the typeface, the kerning and tracking, the size of the type, and more.
The reader shouldn’t even notice the type.
Type size, Type weight, Type style/posture, Line length, Letterspacing, Wordspacing, Linespacing, Justified vs flush left/right, Lowercase/all caps/small caps, Contrast between type and background, Serif vs sans-serif

Other opinions for legibility and readability:
Legibility applies to parts of the text like letters and words and paragraphs. It’s micro-typography. It’s about type’s ability to be easily read, particularly under normal reading conditions.
Readability is most often and more properly used to describe the ease with which written language is read and understood—it concerns the difficulty of the language itself, not its appearance. Factors that affect readability include sentence and word length, and the frequency of uncommon words.
In contrast, legibility describes how easily or comforably a typeset text can be read. It is not connected with contentor language, but rather with the size and appearance of the printed or displayed text.
Readability in English has different meanings. One relates to the ease of reading and understanding a text. Another meaning relates to the ease of visually recognizing letters, numbers, or other symbols on a page, sign, or electronic device. In English, this second meaning is generally referred to as legibility to which another article is dedicated. There is a third meaning related to the sufficiency of embedded documentation in computer-program code, described in the section below, "Readability in computer programming."
Legibility: A Trait, Not Always a Goal
First, not all typefaces are–or should be–created with legibility as a primary design function. Many faces are drawn for the purpose of creating a typographic statement, or for providing a particular spirit or feeling to graphic communication. Some typefaces are just designed to stand out from the crowd. To the degree that a typeface has personality, spirit, or distinction, however, it almost always suffers proportionally on the legibility scale.
Readability is a measure of the comprehensibility or understandability of written text. There are many methods and formulas for determining readability and the related reading age.
Other unknown factors(don’t know which do them belong to?):
sentence and word length, and the frequency of uncommon words.
the size and appearance of the printed or displayed text.
Language