A data value in DHIS2 (core module for aggregated data) is at minimum described by three dimensions, 1) data element, 2) organisation unit, and 3) period, which also forms the core building blocks of the data model. E.g. if you want to know how many children that were immunised for measles in Gerehun CHC in December 2009, the three dimensions to that value are the Data Element "Measles doses given", the Organisation Unit "Gerehun CHC", and the Period "Dec-09". All data values have at least these three dimensions describing What, Where and When.