Structured work establishing the gap between an organisation's current state and the state it requires, in evidence, before any solution is named.
A needs assessment describes two states and measures the distance between them. The current state is what happens today, taken from evidence rather than from what a process document claims; the desired state is what has to be true instead, expressed as an outcome rather than as a tool or a supplier. The distance is the need, and establishing it is the whole purpose of the exercise. Because the assessment stays solution independent, it can be handed to a business case that compares several ways of closing the gap, including doing nothing at all.
See also
The document justifying an investment, evaluating the benefit, cost and risk of alternative options and giving a rationale for the preferred one.
The written output of a needs assessment: the gap an organisation has to close, described with no product, technology or supplier named in it.