A fully unified sample is that portion of a sample audience which was both in-tab and in the sample for all days in a selected set of days. A partially unified sample is that portion of a sample audience which was both in-tab and in the sample for some minimum percentage of a selected set of days.
When a member of an audience sample is in-tab for a given day, the viewing data for that person for the day in question has passed certain validity checks. Otherwise the person is out of tab.
Generally, members of a sample can remain in the sample for only a limited period of time. For the Nielsen sample, this limit is two years.
When doing some types of analysis, it is important that the sample be partially or fully unified, meaning that the portion of the sample over which the analysis is done is both in-tab and in sample for most or all days being considered.