Examples of use
The maj7 or Major 7th chord (sometimes written as △7 to avoid confusion with minor chords - eg. A△7) is usually found on either the I or the IV (1st of 4th) degrees of a major scale.
So in A major, this would be Amaj7 or Dmaj7.
In slightly more jazzy circumstances, you might play a maj7 chord on a more chromatic note of the score, as in this example, in F# major:
Major 7th chords are one of the Diatonic 7th chords
we talk about in the theory lesson here