  Future large-scale galaxy surveys have the potential to become leading probes
for cosmology provided the influence of baryons on the total mass distribution
is understood well enough. As hydrodynamical simulations strongly depend on
details in the feedback implementations, no unique and robust predictions for
baryonic effects currently exist. In this paper we propose a baryonic
correction model that modifies the density field of dark-matter-only $N$-body
simulations to mimic the effects of baryons from any underlying adopted
feedback recipe. The model assumes haloes to consist of 4 components: 1- hot
gas in hydrostatical equilibrium, 2- ejected gas from feedback processes, 3-
central galaxy stars, and 4- adiabatically relaxed dark matter, which all
modify the initial dark-matter-only density profiles. These altered profiles
allow to define a displacement field for particles in $N$-body simulations and
to modify the total density field accordingly. The main advantage of the
baryonic correction model is to connect the total matter density field to the
observable distribution of gas and stars in haloes, making it possible to
parametrise baryonic effects on the matter power spectrum. We show that the
most crucial quantities are the mass fraction of ejected gas and its
corresponding ejection radius. The former controls how strongly baryons
suppress the power spectrum, while the latter provides a measure of the scale
where baryonic effects become important. A comparison with X-ray and
Sunyaev-Zel'dovich cluster observations suggests that baryons suppress wave
modes above $k\sim0.5$ h/Mpc with a maximum suppression of 10-25 percent around
$k\sim 2$ h/Mpc. More detailed observations of the gas in the outskirts of
groups and clusters are required to decrease the large uncertainties of these
numbers.
