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.