In science
Generally, readability is a measure of the accessibility of a piece of writing and/or associated page layout, indicating how effectively it will reach a given reading audience.
Readability is a judgment of how easy a text is to understand for a given established and characterised population.
The understandability of a text is an interaction between the reader whose possible prior knowledge of aspects of the content and the text features would influence the ease with which they access the text, as well as the fixed aspects of the text itself. Presentation factors unrelated to the language of the text also affect readability, for example choice of typeface, text size, layout and colors.
Readability may be assessed by conducting a readability survey, or by application of readability tests which have been established through analysis of readability survey results. Often writers, editors and publishers make an intuitive assessment of readability based on experience, insight into their target audience, and knowledge of a number of rules of thumb, which are often derived from assessment of a number of readability survey results, such as that English textbooks are most readable when laid out with 20% whitespace on the page.
Re-readability - the propensity to again read something previously read after some time - appears to be a criterion dependent upon the reader. Yet, some authors, such as Robert A. Heinlein, appear to be more gifted to produce re-readable works than others as is apparent from the rate of re-printing.