Here are a few definitions from http://www.dictionary.com:
1. morally wrong or bad; immoral; wicked: evil deeds; an evil life.
2. harmful; injurious: evil laws.
3. characterized or accompanied by misfortune or suffering; unfortunate; disastrous: to be fallen on evil days.
4. due to actual or imputed bad conduct or character: an evil reputation.
5. marked by anger, irritability, irascibility, etc.: He is known for his evil disposition.
6. that which is evil; evil quality, intention, or conduct: to choose the lesser of two evils.
7. the force in nature that governs and gives rise to wickedness and sin.
8. the wicked or immoral part of someone or something: The evil in his nature has destroyed the good.
9. harm; mischief; misfortune: to wish one evil.
10. anything causing injury or harm: Tobacco is considered by some to be an evil.
Within the realm of Christianity, let’s use the standard of Christianity’s own truth to define evil.
Strong’s Exhausting Concordance defines evil (as found in Genesis 2 & 3) as “bad or evil (natural or moral)…”
At its most basic, evil could be described as anything opposite of the character of God, since God is good and His character defines good.
Evil can also be defined as behavior which agrees with the character of the devil (John 8:44). This would specifically include lying and everything opposite of the truth; murder and any sort of violence, anger, defamatory comments and actions (character assassination); and stealing or taking what belongs to another by any means – whether this is someone’s money, material possessions, or innocence.