A Limb

Linguistically

1. (astronomy) The apparent visual edge of a celestial body.
2. (on a measuring instrument) The graduated edge of a circle or arc.
3. (botany) The border or upper spreading part of a monopetalous corolla, or of a petal or sepal; blade.

Astrophysics:

The Sun's limb

refers to the outer edge or boundary of the visible solar disk as observed from a particular vantage point. It is the apparent edge of the Sun's disk when viewed against the background of space. Observing the Sun's limb is often essential in solar astronomy, as phenomena such as prominences, spicules, and other solar features are frequently visible near the limb during certain conditions, such as during a solar eclipse or in specialized solar observations.