Beta slippage is a multi-day tracking difference found in leveraged ETF or ETP. Leveraged ETF must rebalance over a predetermined time frame. For example a daily leveraged ETF/ETP rebalances at the market close each day. This means the price movements are calculated on a percentage basis for that day and that day only. Due to rebalancing, the daily leveraged ETF does not track true to its underlying index over a multi-day period. This structural tracking difference caused by the leveraged ETF need to rebalance is defined as beta slippage.
Here is an example of positive Beta Slippage: