The collecting of stamps according to a theme. e.g. Aircraft, Architecture, Animals, Army, Astronomy, Birds, Bridges, Buildings, Butterflies, Cars, Cats, Chess, Cricket, Disney, Dogs, Explorers, Fish, Football, Heraldry, Horses, Famous People, Flowers, Fauna, Gardens, Golf, Insects, Lighthouses, Medical, Royalty, Rugby, Shells, Ships, Space, Sport, Technology, Trains, Writers.