Dimension lumber is softwood lumber that is nominally 2 inches thick and of various lengths and widths. It is the structural softwood lumber used in most wood-based housing construction in North America.
Dimension lumber is produced in various widths (in nominal increments of 2 inches) and various lengths (in increments of 2 feet). The actual cross-sectional dimensions are less than the nominal dimensions, because the wood shrinks as it is dried and it is planed in its final production step. A “two-by-four” is therefore actually 1.5 inches thick and 3.5 inches wide in its finished form (source).