Some Definitions used in Color Characterisation

CIE L*a*b* (CIELAB)
A 3-D color space mathematically derived from CIE xyz chromaticity coordinates resulting in greater perceptual uniformity.
L* = neutral light-dark axis, a* = red/green axis and b* = blue/yellow axis.

dE Color Difference
CIELab  is perceptually uniform color space and the beauty is that it enable us to compute mathematically the distance between 2 colors that represents how close they are in  in the color space. In general, and subjectively speaking,
1 dE: threshold of what is just perceptible to the human eye.
3 dE: detectable by color professionals or an trained eye.
6 dE: often considered acceptable for printed material.

CMYK

Theoretically, black can be produced by mixing magenta, cyan, and yellow which correspond to the three subtractive primaries. To achieve higher quality, CMYK additionally uses black ink to create all colors.
Cyan (also called process blue), Magenta (process red), Yellow (process yellow), Black


lpi
Lines per inch is the number of lines or dots per linear inch in a halftone screen. A 300-600 DPI laser printer can usually only print at an LPI of 50-65, newspapers typically use 85 LPI whereas image-setters and plate-setters print at much higher resolutions and can print up to 200 LPI.

Neutral Gray

A gray which when measured with a calibrated spectrophotometer has zero values for a* and b*, whether or not it looks “neutral” to the eye. In RGB mode, equal amounts of RGB will yield a neutral gray. CMYK has no real solution for an 18% gray other than setting a solid black to 18%. If you plan to print the project using only gray, then the printer color matching becomes critical.  If the grays were balanced properly, the print was naturally accurate because the reproduction process was not adding or taking away color from the original. Otherwise color cast would be introduced to the reproduction of the original.

NPDC

NPDC is a target curve defined along the entire tonal range. G7 uses gray balance as the primary control measurement in that it establishes a  NPDC as a target for the reproduction of highlights and shadows. When neutrality is defined and maintained, color will usually look correct and stay consistent for a print job.

Reference:
GRACoL® 7 Specification for Commercial Printing on  #1 Coated Sheet