Legibility

Legibility is the degree to which writing can be seen and read. Each glyph (individual character) in text has a shape we recognize in reading. The complete set of letters and symbols in a design is called a font (or fount). The choice of font is therefore the first thing which affects legibility.
A second factor in legibility is the setting, the way the type is used in practice. This includes line length, line spacing ("leading"), justification, typestyle, kerning, tracking, point size, etc.