Progressive smouldering means an exothermic oxidation not accompanied by flaming which is self-propagating, i.e. independent of the ignition source. It may or may not be accompanied by incandescence.
Note: In practice it has been found that there is usually a clear distinction between materials which may char under the influence of the ignition source but which do not propagate further (non-progressive) and those where smouldering develops in extent and spreads (progressive).
Source: IMO Resolution MSC.307(88), International Code for Application of Fire Test Procedures, 2010 (2010 FTP Code), 3 December 2010, International Maritime Organization. Regulatory Guidance