When the definite article, the, is combined with an adjective describing a class or group of people, the resulting phrase can act as a noun:

 the poor, the rich, the oppressed, the homeless, the lonely.  

 

The difference between a Collective Noun (which is usually regarded as singular but which can be plural in certain contexts, such as The police and the autorities) and a collective adjective is that the latter is always plural and requires a plural verb:

  • The rural poor have been ignored by the media and society.
  • The richare occasionally socially responsible.
  • The elderly are beginning to demand their rights, and have their voices heard.