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 before then, but up until at least the early 21st century, none were of official or generally accepted knowledge. (DW: Aliens of London/World War Three, The Christmas Invasion)

In 2006, the Ninth 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/World War Three)

On Christmas Day 2006 a live broadcast of a Sycorax was broadcast on the news. It was believed to be a hoax, but when the Sycorax ship hovered over London, with Harriet Jones later speaking to them on behalf of humanity, their existence was confirmed, (DW: The Christmas Invasion) if still denied as having actually happened by some. (DW: The Runaway Bride, TW: Everything Changes)

A differing account states it was the Arcturan Treaty of 2085 that led to humanity having official contact with aliens. (NA: The Dying Days)