An underletting is any arrangement under which you give another person the right to exclusively occupy your property for a period of time, while your own lease remains in place.
It does not matter what the agreement is called or how long it lasts. A tenancy agreement, rental agreement or other arrangement giving someone exclusive use of the property all amount to an underletting.
For example, granting someone exclusive occupation of your property for six months under a tenancy agreement is an underletting, but so too may be granting exclusive occupation for a much shorter period, such as three days.
If your lease requires notice of, or the landlord’s consent to, an underletting, this requirement applies regardless of the length of the arrangement or how it is described.