One problem is that the way the government defines affordable housing is somewhat loose. The 2012 National Planning Policy Framework says that whether a property is classed as affordable is 'determined with regard to local incomes and local house prices', which isn't enormously helpful.
The Housing white paper issued this year isn't much better. There are 40 separate mentions of the phrase 'affordable housing' within the 106-page document, but the definition isn't that clear: 'housing that is provided for sale or rent to those whose needs are not met by the market'.
There are then sub-categories, things like starter homes or homes sold at a discount, and then the added category of intermediate housing.