1. A long ‘tongue’ of relatively cool material (10 000 K) suspended in the much hotter solar corona (c.2 million K). Filaments appear dark when seen silhouetted against the Sun’s disk in Ηα‎ light, but at the limb they appear bright and are termed prominences. Quiescent filaments (the equivalent of quiescent prominences, but on the disk) may show gradual changes, but specific portions or threads within the filament can move more quickly, at speeds of a few kilometres per second. Loop filaments (the disk equivalent of loop prominences) are often associated with large flares. The equivalent of an eruptive prominence is the disappearing filament, sometimes called a disparition brusque, from the French meaning ‘sudden disappearance’. Disappearing or winking filaments may occur as a result of a Moreton wave. An arch filament system is a set of cool loops associated with emerging active regions.

2. A chain-like cluster or supercluster of galaxies. The large-scale structure of galaxy distribution in the Universe is dominated by such features, which may be tens of millions of light years long.