My browser's spelling checker say it isn't. Google results showed no authentic sites apart from wiktionary
Words get into dictionaries, when the dictionary editors find evidence of extensive usage over time. See this Ask the editor video from MW or this explanation by Kory Stamper delivered in a talk show for a story about that.
So, is the word " hypotheticality" being used? (Oh, look, my browser just underlined it as incorrect as well).
Google Books gave 432 results for this word. I haven't checked each and every one of those for false positives and untrustworthy publishers, but here is an example that is good:
Like Reporting, Hypotheticality is based on the notion of authorial detachment, but here the writer detaches him/herself from the world of actuality through the creation of a hypothetical world. Hypotheticality presupposes that the writer is aware of the gap between his/her conceptual world and that of the reader and by means of this device the writer is able to set up a world where there are only two countries, two linguistic theories in order to confine him/herself to those aspects of a situation that will enable him/her to derive a generalization.
from: Advances in Written Text Analysis edited by Malcolm Coulthard, Routledge, 2002.
These alternatives to the directly experienced reality define, respectively, four dimensions of psychological distances - temporal distance, spatial distance, social distance and hypotheticality.
from: Social Psychology: Handbook of Basic Principles edited by Arie W. Kruglanski, Edward Tory Higgins, Guilford Press, 2007
There is another one (found through 'ordinary' Google):
Aspects of the meaning of if … then for older preschoolers: Hypotheticality, entailment, and suppositional processes
This is the title of a scientific article from: Cognitive Development
Volume 7, Issue 2, April–June 1992, Pages 189–212
Psychology isn't my field, but this journal was published by Elsevier, which is a respected publisher for scientific journals.
In the end this Google Ngram shows that the word has appeared here and there since the 30s, but it gained popularity (and if I see correctly the usage tripled) since 1990.
I expected to find it in a dictionary and checked ODO, MW, CDO, LDOCE, AHD, Collins and Macmillan - and found nothing (or I found 'hypothetical' but not 'hypotheticality').
So, is it a word? Based on the fact that it is being used in print, published by reputable sources, I'd say yes. The fact that it is formed in a way nouns are derived from adjectives (as StoneyB noted in his comment) by adding the suffix -ity to the adjective hypothetical, which you will find in many dictionaries, only strengthens this conclusion.
Why isn't it in dictionaries yet? I'm not sure, but this process takes time. If you (and others) use it and keep using it, it just might make it into a dictionary some day.
When talking about conditionals in linguistics and philosophy this word is very commonly used (A conditional is a special type of sentence, typically beginning with if). I see this word on a daily basis in my academic life, and use it too.
Here is a sample of 1,400 odd scholarly articles and books from Googlescholar which use the word hypotheticality.
Yes it is. It is a noun that describing the state of being hypothetical or something of a hypothetical nature. Sources: Your Dictionary
Yes, it is a noun.
Plural form: hypotheticalities
(uncountable) The state or property of being hypothetical.
(countable) Something of hypothetical nature.
It is a new age word like "functionality, usability, drivability, drinkability, playability" and others. In my world it is newspeak, a made up word to eliminate the use of others. Functionality = describing the functions of an item and avoids the descriptions. Usablity = an item is usable. One can merely add "ability" to the end of any word and create a new one. Most people will adopt it as an actual word as it sounds new age and hip. Here's an example..."That trail looks to be an easy walk, it has walkability for most anyone".