What is a Historical Interval
The vSphere Performance data is collected on the ESXi nodes.
The data is kept there for +/- 1 hour, before it is collected by the vCenter, and stored in the vCenter DB. The data that is available on the ESXi node is called the Realtime interval. The Realtime interval has all the metrics.
The Performance data is not kept forever in the vCenter DB. On regular intervals, the so-called aggregation jobs run, and aggregate the available vSphere Performance data. This aggregation takes the data over a number of samples from Historical Interval n, and averages that data. This average data will be stored in the vCenter DB in Historical Interval n+1.
There are four Historical Intervals for the vSphere Performance Data. After the fourth Historical Interval the vSphere Performance Data is removed.