Undertenant is a term used to describe a tenant who rents from another tenant. It is synonymous with words such as subtenant, sublessee, secondary tenant, or occupant. These terms describe someone who rents a property from someone who themselves is renting the property from a landlord or a property owner. Some other related words include lessee, renter, tenant, and occupier. However, it's important to note that these terms do not specifically imply that the tenant has sublet the property to someone else. Overall, the synonyms for undertenant highlight the complex relationships that can exist within tenancy arrangements, and the various roles and responsibilities that tenants may take on.