circular references
a formula that depends, directly or indirectly, on its own result.
A circular reference exists when a cell feeds back into itself: A1 refers to B1, and B1 refers back to A1. There is no fixed order in which to calculate them, so the engine cannot produce a single answer.
Most circular references are mistakes — a total accidentally including its own cell. Some are deliberate, such as an interest calculation that depends on a balance which itself includes the interest; those require iterative calculation, where the engine repeats the computation until the numbers stop moving.
Because the dependency graph decides evaluation order, breaking the cycle usually means introducing an intermediate cell that computes one side independently.