It is an adjective.
Meaning: based on a guess, estimate or theory; not existing in reality.It is also used also in terms of speculative or imaginary.
notionality - noun
notionally - adverb
Synonyms: hypothetical, theoretical, speculative, conjectural,putative, conceptual, abstract,visionary, fanciful, unreal.
Antonyms: actual, genuine, real
The figures published in the newspapers were only notional, since the actual figures could not be obtained.