A flat surface on which outdoor advertisements are posted
This is... [DOC] [TLE] Billboard | Definition of Billboard by Merriam-Webster [PAR] a flat surface (as of a panel, wall, or fence) on which bills are posted; specifically : 
a large panel ... a flat surface on which outdoor advertisements are displayed... [DOC] [TLE] Outdoor advertisements and signs: a guide for advertisers - Gov.uk [PAR] Jun 2, 2007 ... authority's permission for the outdoor advertisement you want to display, this 
booklet .... any hotel, inn or public house, block of flats, club, boarding-house or 
hostel; provided that ... surface area of 2.3 square metres is permitted.