Dex (for decimal exponent) is a unit specifying an amount (e.g., a ratio) as the power of 10 that is relatively near that amount. Another widely used term for this is order of magnitude. A difference is that a dex is sometimes quoted as a fraction whereas an order of magnitude is typically rounded to an integer.
This is the same as the log base 10 of the number. 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.
Within physics and astrophysics, the term dex is sometimes used, but order of magnitude is probably used more.