The process of forecasting how much time and money a piece of work will take. A professional estimate is a range with stated assumptions and risks, not a single confident number.
Budgets and launch dates are built on estimates, so a number given to please the client is the most expensive thing in a project. Being explicit about uncertainty is what makes planning possible.
We estimate in ranges after a short discovery, list what the number assumes, and flag the parts we cannot price yet. For unclear areas we propose a small paid investigation instead of guessing.