I don't know how professionals define it, but it seems to me logical to define an addiction as something which people cannot stop even when it leads to financial, health, or other difficulty that affects normal functioning and/or relationships. That is, the short-term need for a fix overrides universal parts of living a healthy life.

This is not precise, but it seems like an appropriate metric. It's basically the same as defining mental illness in general vs. harmless eccentricity.