The compression ratio is a measure used to describe the efficiency of a compression process, often in engines or data files. It is defined as the ratio of the maximum volume of a cylinder to the minimum volume when the piston is at the top of its stroke. A higher compression ratio typically indicates more efficient fuel use and greater power output in engines, while in data compression, it reflects how much the original data size has been reduced. In the context of data files, the compression ratio is calculated by dividing the size of the original file by the size of the compressed file. For example, if a file is 100 MB and compresses down to 25 MB, the compression ratio is 4:1. This means the data has been reduced to a quarter of its original size, making it easier to store and transmit.