Coarse aggregate
The stone or gravel fraction of concrete, typically ranging from about three eighths of an inch up to an inch and a half depending on the application.
What does coarse aggregate mean?
Larger stone reduces the amount of paste needed, which lowers cost and shrinkage, but it cannot be used where reinforcement is tightly spaced or where the section is thin. Crushed angular stone interlocks better and gains strength more efficiently than smooth river gravel, though it demands more water for the same workability. Maximum size is limited by member thickness and rebar clearance.
In practice
Thin overlays, pumped mixes, and heavily reinforced walls all get smaller stone, which is part of why those mixes cost more per yard.