A fissure, in geological terms, is a long, narrow opening or crack, especially one that allows the passage of fluids such as lava, gas, or water. It typically forms in the earth's crust as a result of the stretching or breaking of rock. Fissures can vary in width and depth, and they may be quite straight or have a more irregular shape.

In the context of a volcanic fissure, it usually appears as a linear crack or series of cracks on the surface of a volcano or the surrounding land. When a volcano erupts through a fissure, the lava emerges from this elongated opening rather than a central vent, leading to a curtain of lava rather than a singular plume. These eruptions can create spectacular flows of lava that spread over a wide area.

Fissures are also found in other geological contexts, like in the drying of mudflats or in glacial ice, where they form crevasses. The key characteristic of a fissure is its elongated, crack-like appearance, which distinguishes it from other types of geological formations.