Caption: A comparison of the spectra obtained by dispersion from a diffraction grating and a prism.
Features:
The slit shape means that diffraction is strong perpendicular to the slit because the slit WIDTH is comparable to the wavelength of the electromagnetic radiation (EMR). On the other hand, diffraction is usually negligible parallel to the slit because the slit LENGTH is large compared to the wavelength of the EMR.
The bright fringes for monochromatic light look like lines, and so are called spectral lines or just lines for a shorthand.
There is always a central bright fringe (i.e., spectral line) for all wavelengths---which is the white beam in the image.
The other bright fringes are numbered in order of angular distance from the central bright fringes: i.e., 1st order, 2nd order, etc. as illustrated in the image. The fringes brightness decreases with order, and so the 1st order is brightest and usually the only one used in spectroscopy.
The central bright fringes can be called the 0th order fringes.
On the other hand, if the incident EMR consists of a discrete set of narrow wavelength bands, each order of the diffraction pattern gets spread into a line spectrum consisting of a set of spectral lines each one formed by one of the narrow wavelength bands. This is situation is NOT illustrated in the image.
A dilute gas gives rise to line spectrum and so we say it HAS a line spectrum.
Usually, when one says spectral line one means one from line spectrum. It can also be called a transition line since it usually results from a transition between energy levels in a microscopic particle (e.g., atom, molecule, ion, or nucleus).
A spectral line has a line wavelength (AKA central line wavelength) which is the central wavelength of the narrow wavelength band that makes up the spectral line. It also has linewidth which is a characteristic width for the narrow wavelength band. Such narrow wavelength bands do NOT have sharp edges, and so the linewidth follows from a conventional definition.
As one can see from the figure, with refraction the shorter the wavelength, the greater the angular deviation of the light ray (i.e., the greater the refraction).