There were four ministries, Truth, Plenty, Peace, and Love. The Ministry of Truth in George Orwell's 1984 is responsible for changing each and every historial or documentary reference such that The Party can change the past and the media as necessary. Winston works for the Ministry of Truth. His job is to change facts in back issues of The Times, such that any "malreports" or "double plus ungood" references no longer exist.
The Ministry of Truth is responsible for the propagation of the infamous saying "Oceania has always been at war with Eastasia", and for general overall deception, deceit, and misinformation, hence the terminology of referring to media which spew falsities or harbor bias about an issue to be analogous to the Ministry of Truth.
See also: Ministry of Love, Ministry of Plenty, Ministry of Peace, 1984 | 
