'Someone who lives in the same postcode area as you. For example, in most cases if your postcode is SE25 1AA, the same postcode area would be another address where the postcode also starts with SE25.'
(via this page)
'For the purposes of delivering items, a neighbour is considered by Royal Mail to be a person who lives within close proximity of the address on the item. They may be a next-door neighbour or someone who lives sufficiently close by.
'Our postmen and women have the flexibility to make an appropriate judgement of which neighbour to approach and indeed whether to approach a neighbour at all. In exercising that judgement they may rely on their familiarity with their walk and pointers of occupation such as previous experience, a car in the drive, or an open window.'