The Near-Infrared Spectrometer and Photometer (NISP) on board the Euclid satellite provides multiband photometry and R ≳ 450 slitless grism spectroscopy in the 950–2020 nm 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 pixel−1 deliver a field-of-view of 0.57 deg2. In photometric mode, NISP reaches a limiting magnitude of ∼ 24.5ABmag in three photometric exposures of about 100 s 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 ∼ 2 × 10−16 erg s−1 cm−2 integrated over two resolution elements of 13.4 Å, in 3× 560 s grism exposures at 1.6 μm (redshifted Hα). Our calibration includes on-ground and in-flight characterisation and monitoring of pixel-based 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 accurate to 5Å or better. NISP is the state-of-the-art instrument in the near-infrared for all science beyond small areas available from HST and JWST – and an enormous advance from any existing instrumentation due to its combination of field size and high throughput of telescope and instrument. During Euclid’s 6-year survey covering 14 000 deg2 of extragalactic sky, NISP will be the backbone for determining distances of more than a billion galaxies. Its near-infrared data will become a rich reference imaging and spectroscopy data set for the coming decades.
