The B altered chord is a 4-note chord consisting of the notes B, D#, A and C.
You can see these notes highlighted in the interactive piano chart below.
The chord itself is often abbreviated as Balt7.
Chords that are a superset of Balt7. The chords include more notes but always B, D#, A and C.
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 Balt7 chord. Feel free to save or share the image as needed.