What is related by fifths?

In Music Theory, "related by fifths" refers to the relationship between musical keys or chords that are a perfect fifth apart. For example, C major and G major are related by fifths, as G is the fifth note in the C major scale. This relationship is significant for understanding chord progressions and modulation in compositions. Additionally, it highlights the circular nature of the circle of fifths, which illustrates the connections between different keys in Western music.