I'd disagree with the accepted answer here.
I'd call this an autotile, and not a 9-slice
"9-Slice" (or 9-patch) is usually used to refer to a system where the content creator slices the image along 4 lines (not necessarily equally-spaced tiles). When rendering a rectangle, the corner slices are displayed at their native size, and the edges/center are stretched (not repeated) to cover the gap.
This is the sense in which Flash, Illustrator, Unity, etc. use the term, so there seems to be good consensus that 9-slice means stretching, not repeating. This is usually used for scaling UI elements like message windows & buttons - and a Google image search turns up this usage almost exclusively.
Image from Adobe docs on 9-slice scaling
Image from the game{closure} docs on 9-slice options
The term I'm more familiar with to describe a set of tiles that can cover an area of any shape or size is "Autotile" (especially in the context of a system that works out/constructs the right tile permutation to use based on adjacency rules, so a level designer doesn't have to hand-pick each tile)
This terminology seems to have been popularized by RPGMaker, which has used a couple of variants on this idea over its generations.
An image search for autotile shows results which seem much closer to what the question describes, and other posts on GameDev.StackExchange describe this as "autotiling" as well.
Image from the docs for the Rotorz Tile System for Unity