What's the difference between grism and grating? In spectroscopic observations, sometimes you meet grism, sometimes grating. 

Both of them could cause light dispersion, but what is the difference?
Transmission gratings on their own introduce chromatic aberration.  This is because they change the effective focal length and do so as a function of wavelength.  The chromatic aberration can be eliminated by introducing a prism of the correct dispersion.  Known as a grism, the grating/prism combination provides an unaberrated image at the 0th order in addition to the spectrum, although the spectrum has fairly low resolution.