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The visible edge of a star, planet, moon, or any other body seen as a distinct disc in the sky. The term limb refers to the edges of the object's disc as seen from Earth, and not to any physical feature or location on the object itself.
Earth's Moon is a somewhat special case, due to the fact that it is tidally locked to the Earth, and therefore features on its surface remain relatively static in relation to its limbs (so it is meaningful to say that, for example, the small lunar sea of Mare Crisium is near the Moon's eastern limb). This is not true of other bodies: for example, a surface feature on Mars or a sunspot on the Sun will appear to emerge from one limb, pass across the disc and eventually be lost to view behind the opposite limb as the object rotates relative to Earth.
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