The term 'Lumber Source' refers to the origin of lumber, which is wood that has been cut into boards or planks for use in construction and other purposes. The crossword solutions for this term include 'MILL' (4 letters), 'Sawmill' (7 letters), 'LOG' (3 letters), 'OAK' (3 letters), and 'Tree' (4 letters). A MILL is a facility where lumber is processed, while a sawmill is a type of MILL specifically designed for cutting lumber. Logs are the raw material used to make lumber, and OAK is a type of tree from which lumber is often sourced. Finally, trees are the source of all lumber.