A solar eclipse in which the apparent size of the Moon is insufficiently large to completely cover the Sun's disc resulting in a ring of sunlight or "ring of fire" around the Moon.
Since the Moon's orbit around the Earth is elliptical the distance between the two bodies varies over the course of a lunar orbit. An annular eclipse occurs when the Moon is close to apogee, the point furthest from the Earth, and therefore appears smaller in the sky than average and not large enough to fully cover the solar disk.