late 14c., dimensioun, 'measurable extent, magnitude measured along a diameter,' from Latin dimensionem (nominative dimensio) 'a measuring,' noun of action from past-participle stem of dimetri 'to measure out,' from dis- (see dis-) + metiri 'to measure,' from PIE root *me- (2) 'to measure.'
A dimension is a measure of a physical variable (without numerical values), while a unit is a way to assign a number or measurement to that dimension. For example, length is a dimension, but it is measured in units of feet (ft) or meters (m). A dimension is the measurement of something in physical space. You might give the painters every dimension of the rooms you want painted, including depth, height, and width. A dimension is a name/value pair that is part of the identity of a metric. You can assign up to 10 dimensions to a metric. Because dimensions are part of the unique identifier for a metric, whenever you add a unique name/value pair to one of your metrics, you are creating a new variation of that metric. A property of space; extension in a given direction: A straight line has one dimension, a parallelogram has two dimensions, and a parallelepiped has three dimensions. The generalization of.
Sense of 'bulk, size, extent, or capacity' is from 1520s, Meaning 'any component of a situation' is from 1929. Related: Dimensional; dimensions.
Dimensions
A line has one dimension (1D), a square has two dimensions (2D), and a cube has three dimensions (3D). In Physics it can also mean any physical measurement such as length, time, mass, etc.
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Updated June 29, 2020
1x Board Sizes
1x4 and 1x6 boards lose 1/4 inch in thickness and 1/2 inch in width before leaving the mill. That means a 1 x 4 board is actually 3/4 inches by 3 1/2 inches. The larger 1x boards (1x8, 1x10, etc.) lose 3/4 inch in width. That means a 1x10 is actually 3/4 inch in thickness by 9 1/4 inches.
2x Board Sizes
2x4 and 2x6 boards lose 1/2 inch in thickness and 1/2 inch in width before leaving the mill. That means a 2 x 4 board is actually 1-1/2 inches by 3-1/2 inches. The larger 2x boards (2x8, 2x10, etc.) lose 3/4 inch in width. That means a 2x10 is actually 1 1/2 inch in thickness by 9 1/4 inches.
See for Yourself
On your next trip to Home Depot or Lowes, take along a tape measure and head over to the dimensional lumber aisles. Measure the actual width and thickness of a pine board (like a 2x4) and then look at the sign above the rack. Hmm. Something doesn't quite match up.
If you can stop someone in the store to help explain the inconsistency, they'll start throwing around a few words like 'nominal sizes' and 'actual sizes' and 'softwood lumber' and 'hardwood lumber.' Hopefully they will take the time to explain exactly what these words mean, which will go a long way towards making you feel more comfortable about buying wood.
For starters, you'll need to know that softwood (like pine, spruce, and fir) is measured and sold one way, and hardwood (like oak, maple, and cherry) is measured and sold in a totally different way. Don't get the two mixed up or you'll really be confused.