  The Near-Infrared Spectrometer and Photometer (NISP) on board the Euclid
satellite provides multiband photometry and R>=450 slitless grism spectroscopy
in the 950-2020nm wavelength range. In this reference article we illuminate the
background of NISP's functional and calibration requirements, describe the
instrument's integral components, and provide all its key properties. We also
sketch the processes needed to understand how NISP operates and is calibrated,
and its technical potentials and limitations. Links to articles providing more
details and technical background are included. NISP's 16 HAWAII-2RG (H2RG)
detectors with a plate scale of 0.3" pix^-1 deliver a field-of-view of
0.57deg^2. In photo mode, NISP reaches a limiting magnitude of ~24.5AB mag in
three photometric exposures of about 100s exposure time, for point sources and
with a signal-to-noise ratio (SNR) of 5. For spectroscopy, NISP's point-source
sensitivity is a SNR = 3.5 detection of an emission line with flux
~2x10^-16erg/s/cm^2 integrated over two resolution elements of 13.4A, in 3x560s
grism exposures at 1.6 mu (redshifted Ha). Our calibration includes on-ground
and in-flight characterisation and monitoring of detector baseline, dark
current, non-linearity, and sensitivity, to guarantee a relative photometric
accuracy of better than 1.5%, and relative spectrophotometry to better than
0.7%. The wavelength calibration must be better than 5A. NISP is the
state-of-the-art instrument in the NIR for all science beyond small areas
available from HST and JWST - and an enormous advance due to its combination of
field size and high throughput of telescope and instrument. During Euclid's
6-year survey covering 14000 deg^2 of extragalactic sky, NISP will be the
backbone for determining distances of more than a billion galaxies. Its NIR
data will become a rich reference imaging and spectroscopy data set for the
coming decades.
