Definition
RSI was created by J. Welles Wilder and usually uses 14 periods. Above 70 is commonly called overbought and below 30 oversold. The 50 line is often used to separate bullish from bearish momentum.
Watch out for
In strong trends RSI can stay above 70 for weeks. Do not short just because RSI is high. Divergence is a warning, not a signal on its own.