Mortar
A mixture of cement, sand, and water with no coarse aggregate, used to bed and bond masonry units rather than to carry structural loads on its own.
What does mortar mean?
Mortar is deliberately weaker and more flexible than the brick or block it joins, so that movement cracks the joint rather than the unit. It is confused with concrete and grout constantly, but the absence of stone is the defining difference. Mortar types trade compressive strength against bond and workability.
In practice
The bag at the home center labeled for laying block is mortar, and using it to pour a footing produces a weak, shrinkage-prone result.