Data reduction is a process where a body of data is reduced to a smaller body of data.
We distinguish among three modes for data reduction
In the "no recovery" mode it is impossible to regenerate the large data from the small one
Using a proper algorithm (hash) it is possible to reduce the large data to small data such that given the small data it would be expectedly difficult to find a large data that would be associated with the small data, using the same hash algorithm. This property is the basis for authentication and non-repudiation crypto protocols.