...
Such instantaneous parameters may, or may not, have undergone post processing, such as being averaged in time. For example, hourly instantaneous values might be averaged over a month, in which case they would be archived in a monthly mean "stream". This would be indicated in other metadata in the GRIB header of the data, as would details of the averaging frequency and period. Even in the case of a monthly mean, these parameters would still be referred to as "instantaneous".
Note that other forecast parameters (not instantaneous parameters) are not strictly valid at the validity time, but are in fact valid throughout a processing period. During this period, the parameter could be accumulated, averaged or the minimum/maximum could be calculated, from the values at all the model time steps during the processing period. For more information, see ERA5 terminology: analysis and forecast; time and steps; instantaneous and accumulated and mean rates and min/max parameters
...