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)

Humanity's first recorded official peaceful first contact with aliens was the Arcturan Treaty of 2085. (NA: The Dying Days)

Other references
In 2007, Gwen Cooper colloquially referred to her kiss with the sex gas-possessed Carys as "first contact with aliens", saying that it wasn't quite what she expected. (TW: Day One)