In music theory, an augmented fourth is an interval that has six half steps 
The augmented fourth requires that:
- The interval must be a fourth interval (four note names between the first and the last).
- The interval must have six half steps.
What does an augmented fourth look like?
Here is an example of a melodic augmented fourth (two music notes in a melody) and a harmonic augmented fourth (in a chord):
How to recognize an augmented fourth?
Rule of music theory: All intervals in a major scale starting with the tonic (degree I) are either major or perfect, and only unison, octave, fourth and fifth are perfect (the others are major).
Example with the interval B♭ / F♯:
Let's take the B flat major scale to have B♭ as the tonic:
From the rule stated above, the interval B♭ / F♮ is a perfect fourth, so the interval B♭ / F♯ is an augmented fourth.
Inversion of the augmented fourth
The inversion of the augmented fourth is the diminished fifth.
Here is an example of an diminished fifth:
Musical examples of augmented fourth
Maria theme from West Side Story by Bernstein
The melody of the theme Maria (from West Side Story) by Bernstein start with an augmented fourth:
Interval identification game
You will find this interval in my Intervals identification game: