  We investigate the physical significance of a new spectral parameter, eta.
This parameter was defined using a Principal Component Analysis of the 2dF
Galaxy Redshift Survey, to retain astrophysical information while minimising
the effect of measurement uncertainties. We find that while eta is correlated
with morphological type, there is a large scatter in this relationship. A
tighter empirical relationship is found between eta and the equivalent width of
the Halpha line, suggesting a connection with the relative star formation rate.
We pursue this connection using spectral synthesis models. Using models in
which the star formation history is parameterised in terms of an exponentially
decreasing function of time, we find that there is a tight correlation between
eta and the ratio of the present to the past-averaged rate of star-formation,
often known as the ``birthrate'' parameter b. This correlation also holds in
models with much more complicated star formation histories, generated by a
semi- analytic model of galaxy formation based upon the hierarchical formation
scenario. There are two possible causes for the tight correlations we find
between eta and b in those galaxies with the most complex star formation
histories; Firstly, the spectra themselves may be degenerate to the actual
long-term star formation history of each galaxy in the optical wavelength range
probed by the 2dFGRS. Secondly, the birthrate parameter b may represent a
physically fundamental quality of galaxy halos -- their over-density relative
to the background density. We conclude that the tight connection with b makes
eta a physically meaningful, as well as convenient and robust, statistic for
galaxy parameterisation and classification. (abridged)
