Some ECMWF parameters are described as being valid at the specified time, the validity time. Sometimes they are referred to as "instantaneous" parameters, and may even have "instantaneous" in their name. However, although they are not really instantaneous because valid at the specified time, they represent an average over time scales equivalent to length of the model time step because they do not represent variability on time scales shorter than this. Some forecast and most, if not all, analysed parameters fall into this "instantaneous" category.
Note, that in addition, such parameters may, or may not, be further averaged in time. For example, they might be averaged over a month from hourly data, 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.
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