Refers to the standard sizes for framing lumber. joists for example are about 2 inches thick by 8, 10, or 12 inches wide, depending on where they're used. Wall studs are usually either 2X4 or 2X6 inches.
hardwood dimension lumber is processed to be used whole in the manufacture of furniture or other products. Softwood dimension lumber consists of boards more than 2 inches thick but less than 5 inches thick. This wood is used in construction and is sold as 2 by 4s, 4 by 8s, or 2 by 10s.
wood products that are sawn from logs. Hardwood dimension lumber is often used in the manufacture of furniture or other products. Softwood dimension lumber is most commonly used in construction, furniture, and other products.
Lumber cut, or S4S, to predetermined specific width (sometimes also to length). 1" x 4", 2" x 4", 2" x 6", 4" x 4" - 8, etc. ( Note: both hardwood and softwood dimension lumber are piece tallied and not measured with a board rule.)