Yes, it does. It's quite literally in the dictionary: https://www.merriam-webster.com/dictionary/headspace

Ok have fun performing psychological experiments on your brain using "informal" merriam webster definitions

You're just being pedantic and grasping at straws. Whether or not we're using the scientific term for an individual's mindset (which is not quantifiable to a high degree of precision but that doesn't meant it's not quantifiable at all, you can at least reduce most 'measurements' to a binary range) is irrelevant to its significance. The comment you replied to specifically said it would be hard to do a rigorous study of.

ok, you can replace that word with "mood and the general train of thoughts at the moment". Happier now?

Everyone knows what it means, and it is way shorter than what I have in the quotes. Pretending it isn't a legitimate word (despite it having an actual entry in the dictionary) doesn't make you look more impressive.

You don’t understand anything when you read or hear the word?