A vehicle provided by a manufacturer on an extended basis to allow an operator to evaluate over a longer period of time than a normal demonstrator. Thus, it is a "seed" allowing a much greater order to be "harvested" from the operator by the manufacturer.
There are LOTS of examples (and there was a thread recently) but in relation to First Group... They took four Streetlites in 2012/3, as well as an Optare Versa and an Optare Solo SR. The Streetlites were subsequently retained by First and led to large orders, and whilst the Optares did lead to some orders being placed, the seed vehicles returned to Optare for disposal.
The individual circumstances e.g. ownership of the vehicles, length of evaluation etc, varies throughout the industry.