Annualised standard deviation of (log-)returns: σ_ann = σ_daily,√(252) under the iid assumption. Distinguish (i) realised vol from historical data (close-to-close, Parkinson, Garman-Klass, Yang-Zhang estimators progressively use intraday range), (ii) implied vol from option prices, (iii) instantaneous vol σ_t from stochastic-vol models. The three are not equal and their differences (variance risk premium = realised - implied) is itself a tradeable risk premium.