In a scale, what is the interval between the first and the third note called?

The interval between the first and the third note of a scale is called a Major Third when the first note is the root of the scale and the third note is a major third above it. To define this more clearly, a Major Third consists of four half steps (or semitones).

For instance, in the C Major scale (C, D, E, F, G, A, B), the first note is C, and the third note is E. Counting the half steps from C to E gives us: C to C# (1), C# to D (2), D to D# (3), and D# to E (4). That makes it a Major Third.

This understanding is critical in music theory because the Major Third is foundational in defining the quality of many chords and scales. In contrast, other choices describe different types of intervals that do not fit the relationship between the first and third notes in a Major scale.