Since you’re new at this, it’s called “.45 Auto”.
Most people still call it the .45 ACP, me for instance. It was .45 ACP when it was invented and it is still the .45 ACP in my mind and in many others as well.
To correct someone on a point this trivial is beyond ridiculous.
.45 A(utomatic) C(olt) P(istol) or .45 auto, same difference.