The chord and the diameter are perpendicular.
Suppose that chord AB is bisected by a diameter, which interserct the chord at point C.
Now, focus on triangles OAC and OBC. They have:
So, they are congruent. In particular, this means that angles ACO and BCO are equal. Since they sum up to 180, each of them must be 90°, so the diameter is perpendicular to the chord.
