Delta-E is a measurement of color difference based on research about the visual distance between two given colors as they appear to the human eye. The method of calculating Delta-E is technically described in several ISO standards. For non-technical practitioners, a quick rule of thumb follows:
< 1ΔE = difference between two colors is not visually perceptible
1 – 2ΔE = barely noticeable difference between two colors can be detected through close observation
2 – 10ΔE = difference between two colors can easily be noticed at a glance
However, these rules are just suggestions. There are some colors, like neutrals, that are so sensitive to change that even a Delta-E of 1 will be visibly apparent. This is why the industry (and Pantone) uses newer Delta-E method called ΔE2000. While still not perfect, the math of ΔE2000 does a better job of matching the way the human vision sees differences in color.