The meaning of "business" is not defined for IHT purposes, so it
has its ordinary meaning which is a trade or profession carried on
IHTA84/S103 (3) says that businesses "includes a business carried on in the exercise of a profession or vocation, but does not include a business carried on otherwise than for gain". The effect of this is to exclude "hobby businesses".
In most cases it will be quite clear whether or not a person was engaged in a business. If there is any doubt you can ask whether or not the Inspector of Taxes had accepted that the deceased had been in business. If the point cannot be resolved reasonably quickly refer the case to your manager for advice.
Similarly where the deceased's description on page 1 of the IHT400, or something else such as a bequest in a Will, indicates that the deceased was in business but no business is disclosed amongst the assets, you should query the position. Do not do this, however, if it is clear that the deceased had retired from business or was in fact only an employee.