The Strangler Pattern, first described by Martin Fowler, is a software architectural pattern that describes a method of gracefully migrating a legacy system incrementally rather than in a big-bang approach. It is named after the Strangler Fig, a type of vine that slowly grows on a tree and eventually replaces it. Similarly, in the software context, the Strangler Pattern involves building a new system around the boundaries of the old system, allowing you to slowly replace parts of the old system with components of the new system over time.
Many Software engineers will be confronted with system migration in their careers; technology is growing faster, and humans need time to adapt and maintain their systems, which can become old sometimes even before they have time to finish it. A strangler pattern is an approach that allows migration without a big bang, which can be very stressful for teams and is often condemned to failure. This pattern is very efficient in the context of large systems because it allows people to gain confidence in the ability to make the migration, providing a multitude of small successes that are much harder to achieve in a one-shot.