Tail risk refers to the risk of an asst moving three standard deviations from its current price on the back of a rare event. They call it tail risk in the crypto market: the risk of an asset moving three standard deviations from its current price on the back of a rare event. Traders fear such an event in bitcoin (BTC) even though the cryptocurrency has been listless around $26,000 since falling over 10% in the week ended Aug. 20. BTC's annualized seven-day historical or realized volatility has cooled to 26% from nearly 60% seen early last week, according to Amberdata.