Personally I would define it as two jobs in a row one year or less.
Why one year?
Well think about it - how long does it take you to start adding value at an employer? One week? One month? So, if you are earning your keep within one month, why does the company care? They always make more than they spend on you. If it takes you 2 years of employment to pay back the employer in terms of revenue that you generate for them, then either they are undercharging for your services or you are plain useless.
This notion of job hopping is two years or less is an outdated mindset. You need to think about why it is considered job hopping, and what is bad about it.
Employers dont like job hopping because they want to believe that you are loyal to the company you work for and will be there forever. Nevermind the fact that most employers are not loyal to their employees - that doesnt figure into the equation.