A formula that refers back to its own cell, directly or through a chain, so it can't resolve.
Try it with AIA circular reference happens when a formula depends on its own result — A1 = A1+1, or a longer loop A1→B1→A1. Excel can't compute a stable value and warns you, unless iterative calculation is intentionally enabled.
Formulas ▸ Error Checking ▸ Circular References lists the offending cells.
Errors