A sublease is renting out space you lease but no longer fully need to another occupier, with your landlord’s consent. It is the standard mid-lease response to surplus space when a break clause is years away.
Hybrid work made subleasing common: organisations consolidate onto fewer floors and sublet the rest. The prerequisite is knowing precisely how much space you can release without crowding your own peak days, which is a utilisation question before it is a legal one.