What Is The Difference Between These "Dielectric" Terms?

Can someone please explain to me what the difference between these terms are?



1. Dielectric constant

2. Relative dielectric constant

3. Dielectric loss



I came across them on this website:



http://www.lsbu.ac.uk/water/microwave.html#pen



Also, I don't really know what "δ" and "ε_r'" on the website are meant to represent.



All and any help would be appreciated.



Thank you.

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