Postcode District
A sub-area of the postcode area, specified by the character sub-string within the first half of a full postcode, which may be numeric, alphabetic or alphanumeric; for example, 42 from MK42 6GH or 1A from W1A 4WW. There are approximately 2 800 postcode districts in Great Britain.
NOTE: there are certain non-geographic districts. In these instances, a district code is allocated to cover all large users in the postcode area.
Description of http://data.ordnancesurvey.co.uk/ontology/postcode/PostcodeDistrict