What is Additionality?

Legal Definition
Additionality is the property of an activity being additional. It is a determination of whether an intervention has an effect, when the intervention is compared to a baseline. 'Interventions' can take a variety of forms, but often include economic incentives. For an example of the application of additionality in environmental crediting markets refer to carbon credits.

Additionality may be evaluated ex post, as is often done in the practice of program evaluation. Or it can be done ex ante, as an initial eligibility screen for issuing credits as part of an environmental or other public goods market. For ex ante applications, additionality is evaluated for proposed activities. A proposed activity is additional if the recognized interventions are deemed to be causing the activity to take place. The occurrence of additionality is determined by assessing whether a proposed activity is distinct from its baseline. A baseline is a prediction of the quantified amount of an input to or output from an activity resulting from the expected future behavior of the actors proposing, and affected by, the proposed activity in the absence of one or more policy interventions, holding all other factors constant (ceteris paribus).

Other working definitions of the term are available here.
-- Wikipedia
Legal Definition
A new input that adds to existing inputs leaving a bigger aggregate.