Interest vs. Usury

Is there an objectively defined difference between charging interest on a loan and usury?

A person I once knew was reluctant to accept a job offer as a loan officer at a financial institution because he feared that he might be guilty of charging usury to some of his future clients.

Someone told me that usury is an "unreasonable" amount of interest. However, it seems as if that's a very subjective definition. What may be "unreasonable" to one person or in one set of circumstances may be "reasonable" to some one else or in a different set of circumstances.

If this person's concept of the difference between the two is the only accurate one, where then does one precisely draw the line between interest and usury?