In modern English we tend to think of evil as being some extreme nastiness that is only found in the worst of humans. In fact, many scholars debate whether or not this brand of evil even exists. Most people would call terrorism evil, but they debate whether or not lesser criminals should be defined in this manner.
The Judeo-Christian view of evil differs significantly from the English definition and utilization of it. In Christian teaching, that which is of God and in His nature is good and anything that isn’t Godly is evil. By this reckoning, even the smallest, most subtle, and seemingly innocuous things would be labeled evil. Music that doesn’t expressly give glory to God, for example, would by definition be evil. Any thought that doesn’t center on God would by nature be evil.
This view will be quite shocking to modern hearers, who often make it a point to be offended by everything from an actual insult to the color of mustard or even the existence of a hot dog. However, the Biblical concept of evil is simply “not good” and all good comes only from God.
It really isn’t even the same concept as what scholars debate today. This type of evil isn’t comparative. According to Jesus, holding a grudge and cursing your neighbor is just as bad as killing your neighbor, so it is important to note here that English words often don’t do ancient concepts much justice.
TL;DR: When we speak of spiritual evil, the definition is simply “Not Godly.”