Visible artifacts that result in rough color transitions in color gradations. Also known as "Quantizing" or "Posterization".
The example below shows the opening of House of Cards with banding present. Note the blue sky.
This issue is usually caused when the bit depth of an output is lower than that of the source. Most video codecs these days can support 10bit and 8bit video. 10bit video allows for 1024 different colours, while 8bit video can only contain 256 colors.
When a 10bit image is converted to an 8bit image the 1024 colors in the source must be mapped to the 256 available in the output it can result in color banding. Usually when this conversion occurs dithering is used to randomize this quantization error and make the banding less visible. However it is not always successful.
If your master source does not exhibit this issue then it's likely to have been introduced during transcoding. Ensure your transcoder is configured correctly and re-transcode.