Final answer:
The phrase 'in retrospect' means looking back or considering the past and it is used to express regret.
Step-by-step explanation:
The phrase 'in retrospect' means looking back or considering the past. It is derived from the combination of 'retro,' which means 'back,' and 'spec,' which means 'to see or look.' In the sentence, 'In retrospect, I probably should have taken better notes on that topic,' the speaker is reflecting on past actions and expressing regret for not taking better notes. Therefore, the best definition of this phrase in this context is 'in regretting the past.'