Unit of measure that calculates and quantifies the difference between two colors -- one a reference color, the other a sample color that attempts to match it -- based on L*a*b* coordinates.
The "E" in "Delta E" comes from the German word "Empfindung," meaning "feeling, sensation. "Delta" comes from the Greek language, and is used in mathematics (as the symbol Δ) to signify an incremental change in a variable, i.e., a difference. So, "Delta E" comes to mean "a difference in sensation."
A Delta E of 1 or less between two colors that are not touching one another is barely perceptible by the average human observer; a Delta E between 3 and 6 is typically considered an acceptable match in commercial reproduction on printing presses. (Note: Human vision is more sensitive to color differences if two colors actually touch each other.)
The higher the Delta E, the greater the difference between the two samples being compared. There are several methods by which to calculate Delta E values, the most common of which are Delta E 1976, Delta E 1994, Delta E CMC, and Delta E 2000. Delta E 2000 is considered to be the most accurate formulation to use for small delta E calculations (<5). Delta E 76 (also abbreviated as "delta E ab", was specified to be used in some printing standards, so its use is still retained when wanting to conform to those standards.
Daylight human vision (a.k.a., photopic vision) is most sensitive to the green region of the color spectrum around 550nm, and least sensitive to colors near the extremes of the visible spectrum (deep blue-purples at one end and deep reds at the other). For that reason, color differences in the latter regions are harder for the average human observer to detect and quantify, making Delta E measurements for those colors possibly less accurate.
For more information:
Delta E (article at CHROMiX)