Black's Law Dictionary, Sixth Edition, p. 483:

dolus.  In the civil law, guile; deceitfulness; malicious fraud.  A fraudulent address or trick used to deceive some one; a fraud.  Any subtle contrivance by words or acts with a design to circumvent.

Such acts or omissions as operate as a deception upon the other party, or violate the just confidence reposed by him. whether there be a deceitful intent (malus animus) or not.

Fraud, willfulness, or intentionality.  In that use it is opposed to culpa, which is negligence merely, in greater or less degree.  The policy of the law may sometimes treat extreme culpa as if it were dolus,upon the maxim culpa dolo comparatur. A person is always liable for dolus producing damage, but not always for culpa producing damage, even though extreme.

[Black's Law Dictionary, Sixth Edition, p. 483]