retroactive /ˌrɛtrəʊˈæktɪv/

• 1. 

  ADJ If a decision or action is retroactive, it is intended to take effect from a date in the past. (决定或行为)有追溯效力的 [正式]

• 2. 

  retroactively ADV [ADV with v]

  例：

  It isn't yet clear whether the new law can actually be applied retroactively.

  这项新法令是否具有追溯力还不清楚。