A Chroma Key is the name commonly given to the backdrop used in the Chroma Keying process. 

A Chroma Key is essentially a backdrop like other accept that it is a specialist color, either Chroma Key Green or Chroma Key Blue, these colors have become common place as they are considered to be the colors least like human skin tone. Sometimes a chrome key area that is built permanently into a studio by painting a wall in a matt finish it referred to a Bluescreen or Greenscreen.

Chroma Keying is a composite layering process that involved filming a subject in front of a Chroma Key and then via computer compositing an additional image where ever the chroma key color appears. 

Chroma Keys must be kept perfectly clean, as any variant to the color will result in the composite image not appearing. In the same way, consideration must be given to the subject being filmed, skin shine & reflection or colored clothing matching the Chroma Key will result in the composite image being shown where its not desired.

The most commonly seen Chroma Key is on the news each night, where the weather presenter stands in front of a Chroma Key, onto which for the viewer is a map of the weather.