According to the notation in [BuG01], the Time_Dimension in figure 1 can be stated here in the form given in [BuG01, page 222-223]:

In general a dimension D with m dimension values can be defined formally as a m tuple (a sorted list of m values):

Consequently, |D| = m defines the number of dimension values of the dimension D. For instance the dimension Time Dimension (see figure 1) with five dimension values (Time ID, Year, Quarter, Month and Day) can be generally characterised as:

|D_{Time Dimension}| = 5.