A Lookback is generally understood to be a post-mortem analysis on a completed project or project phase.  It is performed with an objective of continuous improvement and learning.  At times it is used to uncover the root cause of either favorable or unfavorable variances in outcome vs expectation.
Two specific variants of Lookback analysis are:
Project Lookback:  It can be a “said/did” analysis for a completed Investment  – Did I achieve the economics that I justified the Project on? Variants of this may include fixing pricing at those in effect at the time of Project sanction, or allowing them to reflect actual realizations/current forecasts.
 
Investment Driver Lookback: This is a powerful but seldom used technique for understanding the root cause for higher level variances and identifying systemic bias in estimating.  It is performed as a “said/did” analysis at the Investment driver level for a population of recently completed projects. Did I estimate CapEx correctly? Reserves? Production? OpEx?  It is usually focused on controllable inputs and does not typically consider economic results.
 
See variance analysis for related forms of analysis.