Master Ear Training: 12 Essential Intervals Every Musician Should Know

• Unison or Perfect Unison: Two voices playing the same note.
• Minor Second or Half Step: One step away from the root note.
• Major Second: Two steps away from the root note.
• Minor Third: Three steps away from the root note.
• Major Third: Four steps away from the root note.
• Perfect Fourth: Five steps away from the root note.
• Tritone: Six steps away from the root note.
• Perfect Fifth: Seven steps away from the root note.
• Minor Sixth: Eight steps away from the root note.
• Major Sixth: Nine steps away from the root note.
• Minor Seventh: Ten steps away from the root note.
• Major Seventh: Eleven steps away from the root note.
• Octave: Same note, one octave above or 12 semitones above.

via Master Ear Training: 12 Essential Intervals Every Musician Should Know