What is the deffintion of compression ratio?

Compression is squeezing something so that it occupies a smaller space. Ratio is the proportion between two quantities or numerical values. Click the link for a detailed explanation. Explanation below!

++++

Therefore a Compression Ratio is the ratio between original and compressed volumes of, normally, a gas, such as the fuel-air mixture in a vehicle engine cylinder.

If the starting volume in the cylinder is 100cc and the piston then rises and compresses the gas to 10cc then the compression ratio is 100:10, which we then simplify to 10:1. (Read the colon as the Word "to".)