The D altered chord is a 4-note chord consisting of the notes D, F#, C and Eb.
You can see these notes highlighted in the interactive piano chart below.
The chord itself is often abbreviated as Dalt7.
Chords that are a superset of Dalt7. The chords include more notes but always D, F#, C and Eb.
In case you prefer a non-interactive variant of the chord chart, we've embedded a PNG image below that shows the notes for the the Dalt7 chord. Feel free to save or share the image as needed.