  The Virgo Environmental Survey Tracing Ionised Gas Emission (VESTIGE) is a
blind narrow-band Halpha+[NII] imaging survey carried out with MegaCam at the
Canada-France-Hawaii Telescope. The survey covers the whole Virgo cluster
region from its core to one virial radius (104 deg^2). The sensitivity of the
survey is of f(Halpha) ~ 4 x 10^-17 erg sec-1 cm^-2 (5 sigma detection limit)
for point sources and Sigma (Halpha) ~ 2 x 10^-18 erg sec^-1 cm^-2 arcsec^-2 (1
sigma detection limit at 3 arcsec resolution) for extended sources, making
VESTIGE the deepest and largest blind narrow-band survey of a nearby cluster.
This paper presents the survey in all its technical aspects, including the
survey design, the observing strategy, the achieved sensitivity in both the
narrow-band Halpha+[NII] and in the broad-band r filter used for the stellar
continuum subtraction, the data reduction, calibration, and products, as well
as its status after the first observing semester. We briefly describe the
Halpha properties of galaxies located in a 4x1 deg^2 strip in the core of the
cluster north of M87, where several extended tails of ionised gas are detected.
This paper also lists the main scientific motivations of VESTIGE, which include
the study of the effects of the environment on galaxy evolution, the fate of
the stripped gas in cluster objects, the star formation process in nearby
galaxies of different type and stellar mass, the determination of the Halpha
luminosity function and of the Halpha scaling relations down to ~ 10^6 Mo
stellar mass objects, and the reconstruction of the dynamical structure of the
Virgo cluster. This unique set of data will also be used to study the HII
luminosity function in hundreds of galaxies, the diffuse Halpha+[NII] emission
of the Milky Way at high Galactic latitude, and the properties of emission line
galaxies at high redshift.
