Guess: a conjecture based on little or no evidence
Estimate: a guess made by an expert
― Neandertal's Guide to Cost Estimating, Naval Aviation Systems
All project work is based on probabilities driven by statistical processes. All decisions made on projects for time, cost, and technical performance involve making decision in the presence of uncertanty. To attempt to make those decisions in the absence of estimating the impact of the decisions is foolhardy at best and doomed to failure at worst.