The term sawlog refers to that part of a tree stem that will be processed at a sawmill. This is in contrast to those other parts of the stem that are designated pulpwood. The differences are that sawlogs will be greater in diameter, straighter and have a lower knot frequency.
Sawlogs most often come from the "butt end" of the stem and are the most financially valuable part of the tree.
References
Sawlog Wikipedia(Text) CC BY-SA