The Strangler Pattern (or Strangler Application Pattern or Strangler Fig Pattern) comes from the strangler vine, which envelopes a tree over time, slowly from the top branches down to the root, while killing the tree and replacing it.
In software design, this pattern prescribes a piecemeal replacement of functionality, often in the context of moving a monolith towards a micro service topology.
A first step is to modify the monolith, so that a functionality is moved behind an API (while still being served from the monolith) and then in a second step moving that API outside, into a dedicated (micro) service. This is to be repeated until all functionality is behind a (dedicated) API, in their own (micro) service.