The inability to read.
(Ben Eisen, MPP Graduate 2009)
The percentage of a country's population which is illiterate is an important indicator of the quality of that country's education system, and of its potential for economic development. The United Nations defines the level of illiteracy for particular nations as the proportion of a population who cannot, with understanding, both read and write a short simple statement on his or her everyday life. A person who can only write figures, his or her name or a memorized ritual phrase is also not considered literate.
Literacy is a good measure of educational achievement in developing regions. For young people in developing regions, literacy may be a better measure of education than enrolment since it usually reflects a minimal level of successfully completed schooling. Illiteracy definitions are subject to variation in different countries.
From the United Nations Statistics Division website, http://unstats.un.org/unsd/demographic/products/socind/illiteracy.html