Introduced on day 1
Estimation
Computing a number for an unknown quantity from data - an average, a rate, a difference - together with how uncertain it is. "The IST-AYT no-show rate is 7.8%, 95% CI 7.1% to 8.5%" is an estimate.
In detail
Estimation targets a well-defined quantity (an estimand) rather than an individual outcome. "What is the average no-show rate on IST-AYT this summer?" is estimation; "how many no-shows on ZX104 next Friday?" is prediction.
An estimate without its uncertainty is half an answer. The same 7.8% means something different from 400 flights than from 4, and the interval says how far a decision can rely on it. The figures above are hypothetical.
The estimand decides which data and method are valid. An average over all flights, an average over flights that operated (survivors), and the effect of a policy are three different estimands; a precise estimate of the wrong one does not help.