Sitting Tenant
A sitting tenant is a tenant who is occupying a property when it is sold or transferred. The new owner purchases the property subject to the existing tenancy - the tenancy continues in full force with the new landlord stepping into the shoes of the previous one.
For investors, buying a property with a sitting tenant can have advantages and disadvantages. Advantages: immediate rental income from day one, no void period, no need to find and reference new tenants. Disadvantages: you are bound by the existing tenancy terms, you may not have chosen these tenants yourself, and if the tenancy is periodic you may want to review and update terms.
Sitting tenants on regulated tenancies (a much older form of tenancy, mostly from before 1989) have very strong security of tenure and can be very difficult to remove if needed. Regulated tenancies significantly reduce property value. If a listing describes a "sitting tenant" without specifying the tenancy type, ask your solicitor to confirm whether it is an AST (standard modern tenancy) or a regulated tenancy before proceeding.
Most modern sitting tenants are on Assured Shorthold Tenancies. These continue with the same terms under the new owner.