An estimate is an indication of the value of an unknown quantity based on observed data.
More formally, an estimate is the particular value of an estimator that is obtained from a particular sample of data and used to indicate the value of a parameter.
Example: Suppose the manager of a shop wanted to know the mean expenditure of customers in her shop. She could calculate the average expenditure of the hundreds (or perhaps thousands) of customers who bought goods in her shop, that is, the population mean, or she could use an estimate of this population mean by calculating the mean of a representative sample of customers (a much smaller and less costly venture). If this value was found to be $25, then $25 would be her estimate.