What is the IRS definition of a flip

[Pat Reilly]

I'm unclear on this and have not found a definitive answer through google searches. The IRS allows you to buy real estate with your IRA, with some very specific rules for doing this. However there is one rule which says that you cannot flip property, and I can't find a definition for this. So if I buy a property how long do I have to rent it out before I can sell it and it is not considered a flip? For example, if I buy, rehab, and rent for 3 months then sell, is that considered a flip (or not) by the IRS?