What is a flip for tax purposes?

[Robert H Hilbert]

I just read in Han and Macfarlane's tax book that profits from flips get taxed as ordinary gains, not capital gains. What is defined as a flip?  If we merely paint and cleanup and maybe replace carpets for less than $5K, and resell a month later  is that a flip?  What if we do the same work and rent it for a year and then sell, does it still get taxed as a flip?  (I understand that I must claim the expenses the same year as the sale.)