  AG Peg is known as the slowest symbiotic nova, which experienced its
nova-like outburst around 1850. After 165 years, during June of 2015, it
erupted again showing characteristics of the Z And-type outburst. The primary
objective is to determine basic characteristics, the nature and type of the
2015 outburst of AG Peg. We achieved this aim by modelling the spectral energy
distribution using low-resolution spectroscopy (330-750 nm), medium-resolution
spectroscopy (420-720 nm; R=11000), and $UBVR_{\rm C}I_{\rm C}$ photometry
covering the 2015 outburst with a high cadence. Optical observations were
complemented with the archival HST and FUSE spectra from the preceding
quiescence. During the outburst, the luminosity of the hot component was in the
range of 2-11$\times 10^{37}(d/0.8{\rm kpc})^2$ erg/s. To generate the maximum
luminosity the white dwarf (WD) had to accrete at $\sim 3\times 10^{-7}$
M$_{\odot}yr^{-1}$, which exceeds the stable-burning limit and thus led to
blowing optically thick wind from the WD. We determined its mass-loss rate to a
few $\times 10^{-6}$ M$_{\odot}yr^{-1}$. At the high temperature of the
ionising source, $1.5-2.3\times 10^5$ K, the wind converted a fraction of the
WD's photospheric radiation into the nebular emission that dominated the
optical. A one order of magnitude increase of the emission measure, from a few
$\times 10^{59}(d/0.8 {\rm kpc})^2$ cm$^{-3}$ during quiescence, to a few
$\times 10^{60}(d/0.8\,{\rm kpc})^2$ cm$^{-3}$ during the outburst, caused a 2
mag brightening in the LC, which is classified as the Z And-type of the
outburst. The very high nebular emission and the presence of a disk-like HI
region encompassing the WD, as indicated by a significant broadening and high
flux of the Raman-scattered OVI 6825 \AA\ line during the outburst, is
consistent with the ionisation structure of hot components in symbiotic stars
during active phases.
