The ease in which text can be understood (based on content, not formatting). Readability is not to be confused with legibility, which is the ease with which text can be read (based on formatting, not content).
- http://psychology.wichita.edu/surl/usabilitynews/72/LineLength.asp - study on "The Effects of Line Length on Reading Online News", A. Dawn Shaikh, Usability News July 2005, Vol. 7 Issue 2