Edge-on galaxy
Also called: dust lane.
A spiral galaxy seen exactly side-on: a thin streak of light, often split by a dark lane of dust.
When we see a spiral's disk side-on, its light piles up into a thin spindle, which makes edge-on galaxies easier to spot than face-on ones of the same brightness. The disk's dust shows as a dark lane along the middle.
NGC 4565 (the Needle Galaxy), NGC 891 and M 104 (the Sombrero) are the showpieces.
At the eyepiece
Look for the dust lane with averted vision at medium-high power; it's one of the most satisfying details in deep-sky observing.