Ideally, the base metals and filler metal melt and fuse into each other to create a weld joint.
Sometimes, however, the extra filler metal is added into the weld pool, and it does not mix properly with the base metal, allowing it to flow outside the weldment. This is known as welding overlap.
An overflowing swimming pool is a good analogy since we are already talking about a pool. When surplus water exceeds a pool’s capacity, it flows out of the pool and onto surfaces where it should not be.
Welding overlap is extra filler material that spreads out around the bead. Technically speaking, welding overlap is extra filler material that protrudes beyond the weld toe or root.