For me, a traditional knife (folder or fixed blade) is of a style that does not follow the current trend of tacical designs on the market today. They can be classic stockmans or trappers still made by Camillus, Boker, Case, Schrade and others. Even relatively modern knife designs like the Buck 110 look more like a traditional folding knife when paired with a modern one-hand opening tactical folder.
Outdoor/Hunting knives like those made by Grohmann, Marbles, Muela and others have a traditional look about them which is enhanced by the use of natural materials such as wood, stag, bone or horn. Organic materials are a hallmark of traditional knives, but just because a knife has wood scales, does not always mean it's a traditional knife design (example: Benchmade 690). It must follow a specific pattern that has been relatively unchanged for years.
Traditional knives that have been relatively unchanged throughout the years are also those of various cultures which have developed a rich history among it's people, for example: the Spanish Navaja and the Finnish Puukko. These types of knives have been around for centuries and will continue to survive despite the recent "tactical" knife movement.
High-Tech folders and fixed blades with titanium, carbon fibre, G-10 or thermoplastics have their place, but few can attain the charm or the legacy of a traditional knife.