A reference period is the time period for which statistical results are collected or calculated and to which, as a result, these values refer. The time period may be either a calendar year (reference year), a fiscal year, a semester, a quarter, a month and even a day.
Some data, like population variables, relate to one specific time, a reference time point (e.g. a specific day, population on the 1st of January).
The reference period should be distinguished from the publication time, the period or point in time at which the statistical data are published. The publication year of statistical results may be significantly later than the reference year for which they were collected.