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Dark Nebula - A dark nebula is a large molecular cloud which appears as a star-poor region where the dust of interstellar medium seems to be concentrated.
Dark nebulae can be seen if they obscure part of an emission or reflection nebula (eg. the Horsehead Nebula) or if they block out background stars (eg. the Coalsack Nebula).
The form of such dark clouds is very irregular: they have no clearly defined outer boundaries and sometimes take on convoluted serpentine shapes.

Electron - The electron is a fundamental subatomic particle that carries a negative electric charge. It is a spin-½ lepton that participates in electromagnetic interactions, and its mass is less than one thousandth of that of the smallest atom. Its electric charge is defined by convention to be negative, with a value of −1 in atomic units. Together with atomic nuclei, electrons make up atoms; their interaction with adjacent nuclei is the main cause of chemical bonding.

Globular Cluster - A globular cluster is a spherical collection of stars that orbits a galaxy core as a satellite. Globular clusters are very tightly bound by gravity, which gives them their spherical shape, and relatively high stellar density towards their core. Globular clusters contain considerably more stars than the less dense galactic, or open clusters. Globular clusters are generally composed of hundreds of thousands of old stars, but someglobular clusters have a mass of several million solar masses.
A globular cluster is sometimes known more simply as globular.

Interstellar Medium - The interstellar medium (or ISM) is the name astronomers give to the tenuous gas and dust that pervade interstellar space. The ISM consists of an extremely dilute (by terrestrial standards) plasma, gas and dust, consisting of a mixture of ions, atoms, molecules, larger dust grains, electromagnetic radiation, cosmic rays, and magnetic fields. The matter consists of about 99% gas and 1% dust by mass. It fills interstellar space. This mixture is usually extremely tenuous, with typical densities ranging from a few hundred to a few hundred million particles per cubic meter.

Light year - (or light-year, or lightyear), symbol ly. Is the distance light travels in one year: about 9.461×10¹⁵ meters. More specifically, a light year is defined as the distance that a photon would travel, in free space and infinitely distant from any gravitational or magnetic fields, in one Julian year (365.25 days of 86,400 seconds each). Since the speed of light in a vacuum is exactly 299,792,458 m/s, by the definition of meters, one light year is exactly equal to 9,460,730,472,580,800 m.
The light year is often used to measure distances to stars. In astronomy, the preferred unit of measurement for such distances is the parsec. This is equal to approximately 3.26 light years.

Magnitude, absolute - Absolute magnitude is the apparent magnitude, m, an object would have if it were at a standard distance away from us. It allows the overall brightnesses of objects to be compared without regard to distance.
The absolute magnitude uses the same convention as the visual magnitude, with a ~2.512 difference in brightness between step rates (because 2.512⁵ ≈ 100). The Milky Way, for example, has an absolute magnitude of about -20.5. So a quasar at an absolute magnitude of -25.5 is 100 times brighter than our galaxy. If this particular quasar and our galaxy could be seen side by side at the same distance, the quasar would be 5 magnitudes (or 100 times) brighter than our galaxy.

Magnitude, apparent (or visual) -  The apparent magnitude (m) of a star, planet or other celestial body is a measure of its apparent brightness as seen by an observer on Earth. The brighter the object appears, the lower the numerical value of its magnitude. More...

Pulsar - A very regularly pulsating source of radiation, which almost certainly originates from a rotating neutron star. The periods range from 1.56 milliseconds to 4 seconds and can be measured to accuracies of, typically, one part in 10 million.

Quasar - A quasar (contraction of QUASi-stellAR radio source) is a source of electromagnetic energy, including light. Quasars all show a very high redshift meaning that they are very distant. To be observable at that distance, the energy output of quasars must dwarf that of almost every known astrophysical phenomenon with the exception of comparatively short-lived supernovae and gamma-ray bursts. They may readily release energy in levels equal to the output of hundreds of average galaxies combined. The output of light is equivalent to one trillion suns.
Most quasars look like single points of light. Some quasars display rapid changes in luminosity, which implies that they are relatively small. More...

Star - A star is a massive, luminous ball of plasma. Stars group together to form galaxies, and they dominate the visible universe. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth, including daylight. Other stars are visible in the night sky, when they are not outshone by the Sun. A star shines because nuclear fusion in its core releases energy which traverses the star's interior and then radiates into outer space. Without stars, life and most atomic elements present in the Universe would not exist. More...

Supernova - A star that has exploded violently and is observed to brighten temporarily to an absolute magnitude brighter than about -15. A supernova explosion blows off all or most of the star's material at high velocity. The debris consists of an expanding gas shell and possibly a compact stellar object (a neutron star or a black hole), which is the original star's collapsed core.

Text verified and adapted from Wikipedia (http://en.wikipedia.org/wiki/Main_Page).