1. Someone who is unable to read or write: Susan was an illiterate who did not know the alphabet, let alone competent enough to peruse the contents of a text.
2. Anyone who has or demonstrates having very little or no education: In some parts of the world there are many illiterates who have had little or no formal or proper schooling.

Someone whose reading and writing abilities are inadequately developed to meet everyday needs: A functional illiterate does not have the skill to manage daily living and employment tasks which require reading skills beyond the basic level.

A functional illiterate can also be a foreigner who is not able to read or write in the native language where he or she lives.