Stopping criterion
The rule that decides when an iteration has reached sufficient accuracy.
Definition
The usual criteria combine the increment (absolute or relative), the residual and a maximum number of iterations as a safety net. Each can mislead on its own: the increment in slow iterations, the residual in ill-conditioned problems.
How it is used
In practice one requires both a small increment and a small residual, with tolerances consistent with machine epsilon and with the data.