  We extend Dain's construction of a geometric invariant characterising static
initial data sets for the vacuum Einstein field equations to situations with a
non-vanishing extrinsic curvature. This invariant gives a measure of how much
the initial data sets deviates from stationarity. In particular, it vanishes if
and only if the initial data set is stationary. Thus, the invariant provides a
quantification of the amount of gravitational radiation contained in the
initial data set.
