Staghorns will have long thick branches looking like a deer or stag antlers. Not all acropora are stags your looking at Acropora cervicornis when your looking at stags.
Depends who you ask, author Cardene Wallace, who wrote a great ID book called " staghorn corals of the world" refers to all Acropora as staghorn corals.
In the hobby, we generally refer to species like muricata/ formosa, lovelli, hoeksemai, loisettae, grandis, copiosa, robusta, aborhoensis, yongei as "stags"