Because I’m running over the term imputation for the third time, it’s time to have a blog posting as a reminder.
Wikipedia defines imputation as
the substitution of some value for missing data. When substituting for a data point, it is known as “unit imputation”; when substituting for a component of a data point, it is known as “item imputation”. Once all missing values have been imputed, the dataset can then be analysed using standard techniques for complete data. The analysis should ideally take into account that there is a greater degree of uncertainty than if the imputed values had actually been observed, however, and this generally requires some modification of the standard complete-data analysis methods. Many imputation techniques are available.