What is a 0.648.5?

The expression "0.648.5" appears to be a typographical error, as it combines numbers and a decimal point in a way that is not standard. If you meant to write "0.64 raised to the power of 8.5" (0.64^8.5), it refers to the mathematical operation of exponentiation. In this case, 0.64 is the base, and 8.5 is the exponent, which results in a value less than 1. If this interpretation is incorrect, please clarify your question.