Expansion joint
A gap filled with compressible material that allows concrete to expand without pushing against adjacent construction. Far less commonly needed in flatwork than people assume.
What does expansion joint mean?
Because concrete shrinks more over its life than it expands, most joints in slabs are actually contraction or isolation joints rather than true expansion joints. Genuine expansion joints matter in long pavements, bridges, and where concrete abuts a structure it could damage by pushing. Overusing them in slabs creates unnecessary maintenance points that collect debris and fail.
In practice
The felt strip a contractor calls an expansion joint between a driveway and a garage slab is functioning as an isolation joint.