Dex (for Decimal Exponent) is a unit specifying a number or ratio as the power of 10 that equals (or is relatively near) that number. In other words, the log base 10 of the number, aka its Order Of Magnitude. For example, 100 could be described as 2 dex, or two numbers that differ by a factor of 1000 could be said to differ by 3 dex.
The term "dex" is used in physics and astrophysics.
H-R Diagram (HRD)