A solar prominence seen against the bright solar disk is called a filament. It appears dark because its relatively cool, dense plasma blocks some of the brighter light coming from the photosphere beneath it.
Prominences and filaments are essentially the same kind of structure: material held above the Sun by magnetic fields, often along boundaries between regions of opposite magnetic polarity. The name depends mainly on viewing angle. Seen projecting beyond the Sun’s edge against the dark sky, it is called a prominence; seen silhouetted across the disk, it is called a filament.
These structures can stretch hundreds of thousands of kilometres and may remain stable for weeks or months. If their supporting magnetic fields become unstable, they can erupt and contribute to a coronal mass ejection, sending magnetized plasma into space.
A filament is not a coronal loop, although both are shaped by solar magnetism. Filaments are also much cooler than the surrounding corona, which helps make them visible as dark ribbons.