A subtenancy is the renting arrangement that exists when a tenant rents their place, or part of it, out to somebody else. Note that the word is about the arrangement, not the person — the person is a 'subtenant'. So there are two rentings stacked up: the landlord rents to the tenant, and the tenant rents on to the subtenant, and that second one is the subtenancy. It is a legal and business word; you meet it in contracts, in rental law, and in landlord notices — 'the lease forbids any subtenancy without written permission'. Ordinary people almost never say it; they say 'subletting' or just 'renting a room from someone'. It matters legally because the subtenant usually has no direct agreement with the owner, so if the main tenancy ends, the subtenancy normally falls with it. The plural is 'subtenancies'.

轉租關係、次承租權（房客再把房子租給別人的那層租約）