Atrium

NCC

means a space within a building that connects 2 or more storeys and—

(a) is enclosed at the top by a floor or roof (including a glazed roof structure); and
(b) includes any adjacent part of the building not separated by an appropriate barrier to fire; but
(c) does not include a stairwell, rampwell or the space within a shaft; and
(d) for the purposes of (a) a space is considered enclosed if the area of the enclosing floor or roof is greater than 50% of the area of the space, measured in plan, of any of the storeys connected by the space.

Standards

as defined in Part G3 of the NCC

Source - AS 1668.1:2015: The use of ventilation and air conditioning in buildings

HB50

means a space within a building that connects 2 or more storeys, and:

(a) is wholly or substantially enclosed at the top by a floor or roof (including a glazed roof structure); and
(b) includes any adjacent part of the building not separated by an appropriate barrier to fire; but
(c) does not include a stairwell, rampwell or the space within a shaft

Source - BCA Vol 1 terminology