What is "dimensional lumber"

Lumber \Lum"ber\, n. [Prob. fr. Lombard, the Lombards being the money lenders and pawnbrokers of the Middle Ages. A lumber room was, according to Trench, originally a Lombard room, or room where the Lombard pawnbroker stored his pledges. See Lombard.]

1. A pawnbroker's shop, or room for storing articles put in pawn; hence, a pledge, or pawn. [Obs.]

   They put all the little plate they had in the lumber, which is pawning it, till the ships came.
   --Lady Murray.

2. Old or refuse household stuff; things cumbrous, or bulky and useless, or of small value.

3. Timber sawed or split into the form of beams, joists, boards, planks, staves, hoops, etc.; esp., that which is smaller than heavy timber. [U.S.]

   Lumber kiln, a room in which timber or lumber is dried by artificial heat. [U.S.]

   Lumber room, a room in which unused furniture or other lumber is kept. [U.S.]

   Lumber wagon, a heavy rough wagon, without springs, used for general farmwork, etc.

   dimensional lumber, lumber, usually of pine, which is sold as beams or planks having a specified nominal cross-section, usually in inches, such a two-by-four, two-by-six, four-by-four, etc.

n. (context US English) Lumber that is finished/planed and cut to standardized width and depth specified in inches.