compression ratio

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun In an internal-combustion engine, the ratio of the volume of the combustion chamber measured before and after a compression stroke.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun automotive The ratio of the volume between the cylinder head of an internal combustion engine and the piston before and after the compression stroke
• noun computing The ratio of the size of compressed data after the execution of some compression algorithm to the size of the original data.