What is an estimated digit?

Answer 1

Suppose you have a variable whose exact value is not known. Repeated estimations give values which match up to a certain degree but beyond that they are different. You could take some sort of [weighted] average of the leading digits of these different parts and your estimate for the variable wold be the matched part with the estimated digit appended.

For example, if you Google "mass of proton" the WIkipedia site give it as ‎1.672621898(21)×10−27 kg. The (21) - in parentheses - indicates that those two digits are estimated.