  We have structurally characterized the liquid crystal phase that appears as
an intermediate state when a dielectric nematic, having polar disorder of its
molecular dipoles, transitions to the almost perfectly polar-ordered
ferroelectric nematic. This intermediate phase, which fills a 100-year-old void
in the taxonomy of smectics and which we term the "smectic $Z_A$", is
antiferroelectric, with the nematic director and polarization oriented parallel
to smectic layer planes, and the polarization alternating in sign from layer to
layer. The period of this polarization wave ($\sim$180 A) is mesoscopic,
corresponding to $\sim$40 molecules side-by-side, indicating that this lamellar
structure is collectively stabilized. A Landau free energy, originally
formulated to model incommensurate antiferroelectricity in crystals, describes
the key features of the nematic-Sm$Z_A$-ferroelectric nematic phase sequence.
