I. ˌBig ˈBrother¹
a character in the novel Nineteen Eighty-Four by George Orwell. Big Brother is the leader of the state, and although no one has ever met him there are pictures of him everywhere with the message ‘Big Brother is watching you’. People now use the expression ‘Big Brother’ to describe any government or organization that has complete power, allows no freedom, and carefully watches what people are doing:
Increasingly, the state is taking a big brother role in this area.

[TahlilGaran] Dictionary of Contemporary English ▲

II. Big Brother²
a television programme in the UK, US, and several other countries, in which several people are chosen to live in a house together. Their actions and conversations are then filmed and shown on television. Once a week, the people who watch the programme vote on who they think should leave the house. This programme was very popular and some of the people who took part in it became famous.

[TahlilGaran] Dictionary of Contemporary English ▲

Big Brother
1.
a government or a large organization which tries to control every part of people's lives and to know everything about them.
Many people are concerned about Big Brother having computer files on them to which they do not have access. (always before noun)

2.
Big Brother - a government or a large organization which tries to control every part of people's lives and to know everything about them.
Employees have complained because of the 'Big Brother' approach of the new security measures.

[TahlilGaran] English Idioms Dictionary ▲