If you buy today and sell after 3 days it is short term. There is no clear definition. Normally 1 year is considered short-term and beyond 1 year is long term. That may not exactly be correct, but that is more for taxation purposes. Actually, long-term investing is about holding on for a period of 10-12 years. Actually, even selling in 2-3 years is a kind of short-term investing only. If you are a trader, then you can also look at time periods of 1 day, few days, few weeks etc.