Legibility is the ease with which a reader can decode symbols. In addition to written language, it can also refer to behaviour or architecture, for example. From the perspective of communication research, it can be described as a measure of the permeability of a communication channel. A large number of known factors can affect legibility.

In everyday language, legibility is commonly used as a synonym for readability. In graphic design, however, legibility is often distinguished from readability. Readability is the ease with which a reader can follow and understand words, sentences and paragraphs. While legibility usually refers to the visual clarity of individual symbols, readability is more about their arrangement or even the choice of words. Legibility is a component of readability.

The legibility of text is most often examined by controlled deterioration of viewing conditions and determination of threshold detection.

Not all writing benefits from optimizing for legibility. Texts that are supposed to be eye-catching or whose appearance is supposed to hold certain connotations could deliberately deviate from easy legibility for these purposes. Corresponding typefaces are called display typefaces.