Deduplication
Storing only one copy of repeated data blocks to reduce backup storage consumption.
Backups are enormously repetitive, since the same operating system files exist on every server and most documents change little between runs. Deduplication stores each unique block once and references it.
Ratios of ten to one or better are common, which is what makes long retention affordable. It does add processing overhead and makes the storage more complex to recover from.
Where this comes up
Deduplication sits inside our backup and disaster recovery work. If you are trying to work out what this means for your own environment rather than in the abstract, that is the page worth reading next, and a short call will get you a straight answer faster than either.