Blue-green deployment is a technique that's used to reduce downtime to deploy a new version of the application. Martin Fowler described this technique at the beginning of 2010 on his site at https://martinfowler.com/bliki/BlueGreenDeployment.html. The following is a quote taken from this site:
"One of the challenges with automating deployment is the cut-over itself, taking software from the final stage of testing to live production. You usually need to do this quickly in order to minimize downtime. The blue-green deployment approach does this by ensuring you have two production environments, as identical as possible. At any time one of them, let's say blue for the example, is live. As you prepare a new release of your software you do your final stage of testing in the green environment. Once the software is working in the green environment...