First contact

First contact was the ambiguous term used to describe a planet's first official contact with alien life.

Human first contact
The Human race had come into contact with many alien races in the past, but none of official or accepted knowledge. The Doctor had claimed that he had perfect knowledge of Human history, but was frequently surprised to find himself involved in the events of it. (DW: Aliens of London) Given that time was later described as mostly in flux, it is possible that there was never any official first contact.

In 2006, the Doctor witnessed an alien ship crash into Big Ben. He believed this to be the first contact for Humans, on a large and newsworthy scale. It was indeed contact with the Slitheen family. However, it was widely believed to be a hoax. (DW: Aliens of London)

On Christmas Day 2007 a live broadcast of a Sycorax was broadcast live on BBC 1. It was believed to be a hoax, but when the Sycorax ship hovered over London, their existence was confirmed. (DW: The Christmas Invasion)

Humanity's first official diplomatic contact with aliens (as opposed to an invasion) was the Arcturan Treaty of 2085. (NA: The Dying Days)