Is there a threshold above which a matrix is ill-conditioned?

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My matrix has a condition number of $45.678$. Is it an ill-conditioned one?

Is there a threshold above which a matrix is ill-conditioned?

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There is no sharp boundary between "well-conditioned" and "ill-conditioned". Your matrix is better-conditioned than one with $456.78$ but worse than one with $4.5678$. Whether you would consider $45.678$ "ill-conditioned" might depend on the particular application. But for most purposes I would think $45.678$ is not terribly bad. It might cause you to lose up to $2$ significant figures of accuracy (i.e. your results may have $2$ fewer correct digits than the data), but assuming the data started out with much more than $2$ significant digits it won't make the results completely meaningless.