Fraud: A form of theft enacted by making a trade based on a lie, and keeping the property in spite of the property transfer conditions not being satisfied.
Fraud as theft involves a trade where the property is actually transferred from one to the other, but the right to the property has not. The transfer of that right to the property is conditional based on the terms of the exchange. If I say I'll buy your watch for $20, and you say okay and hand me the watch, I may have physical possession of it, but I do not own it. I must provide the $20, or I am guilty of theft. Fraud involves creating a temporary illusion that the conditions have been met, in order to gain physical possession of the property. Since the conditions have not truly been met, it is just another form of theft.
Fraud is often viewed as a more general lying for benefit. While this may be a useful concept, it needs to be distinguished between fraud as a form of theft. The latter is a form of force, involving an actual violation of another's property rights. Just lying to gain value is not enough here. While it may be unethical to lie, it is not a violation of a person's rights.