Informal Logic: The logic concerned with natural language argumentation. According to the authors, informal logic is the branch of logic whose task is to develop non-rigid formal standards, criteria, procedures for the analysis, interpretation, evaluation, criticism and construction of argumentation in everyday discourse and in disciplined inquiry.
Source: ‘What is Good? What is Bad? The Value of All Values across Time, Place and Theories’ by John McMurtry, Philosophy and World Problems, Volume I-III, UNESCO in partnership with Encyclopedia of Life Support Systems: Oxford, 2004-11.