quantitative systems description parameter temporal measure Fundamental quantity of the measuring system used to sequence events, to compare the durations of events and the intervals between them, and to quantify the motions or the transformation of entities. The SI base unit for time is the SI second. The second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.