A nominal width is a width which is somewhat less than the stated width. For example, a 2" X 12" board with a 0.75" nominal width has an actual width of 11.25".
Note that:
1. Lumber and timber/beam dimensions (width and height/thickness) are most commonly nominal unless specifically noted as being full dimension or "net". Many Trestlewood products are targeted at 0.5" nominal for stated dimensions of under 6" and 0.75" nominal for stated dimensions of 6" and above.
2. Reclaimed wood dimensions generally vary somewhat from piece to piece -> The amount by which the width of a list of material is nominal is best thought of as an average.