What type of product is referred to as being made mostly from lumber cut into boards from a tree trunk, without wood by-products?

The product described in the question is solid wood. Solid wood is directly derived from timber, which means it consists entirely of the natural wood material from a tree trunk that has been cut into boards without including any wood by-products. This type of material maintains the integrity of the wood's natural fibers, providing strength, durability, and aesthetic qualities typically desired in furniture and construction applications.

Engineered wood, on the other hand, combines wood fibers, strands, or veneers with adhesives, resulting in products such as laminate or composite materials, which differ from solid wood in their structure and production process. Particle board is made from wood chips, shavings, and sawdust, which are compressed and bonded together, making it distinct from solid wood as it incorporates numerous wood by-products. Plywood consists of thin layers or "plies" of wood veneer glued together, which again differs from solid wood due to its layered construction and the use of veneers.

Thus, solid wood is the correct answer as it directly refers to lumber cut from trees, free from the inclusion of any wood by-products.