Define Parsec.

Explanation

**Step-by-Step Solution:**

1. **Definition of Parsec**: A parsec is a unit of length used to measure astronomical distances. It is particularly useful for measuring distances between stars and other celestial objects.

2. **Understanding the Concept**: The term "parsec" is derived from "parallax" and "arcsecond." It relates to the method of measuring distances based on the apparent shift in position of an object when viewed from different locations.

3. **Parallax Angle**: To define a parsec, consider the Earth’s orbit around the Sun. If we observe a star from two different positions of the Earth (6 months apart), we can measure the angle of parallax (θ) that the star appears to move against the background of more distant stars.

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