Informal logic has expanded the concept of an 'argument' beyond that presented traditionally by formal logicians-to include arguments as encountered in 'real-life'. Existent definitions of argument structure are argued to be inadequate by failing to fully recognise that, ultimately, arguments have a human source. Accordingly, a new definition is proposed which appeals to relevant cognitive and behavioural factors. THe definition retains some traditional concepts, but introduces the term 'supportive' as a modification to 'premiss'. The concept of a 'persuader' is also developed. The definition is argued to capture more fully the intricacies, subtleties and rich diversity of informal arguments.