With respect to option contracts and other types of derivatives, it is the partial period of time during which the holder (the long) is allowed to exercise the contract using the most favorable terms over the contract’s life. For example, in a partial lookback option the holder can exercise the option using the best price that has been reached during the partial lookback period. In case of a lookback call (i.e., a partial lookback call) on an index, the holder would receive, at expiration, the difference between the strike price and the highest price the index has reached during partial lookback period.