The quality of paying close attention to something or someone; being alert and focused on what is happening around you.
From attentive (Latin attendere: ad- 'to' + tendere 'to stretch') + -ness suffix forming nouns. The root refers to 'stretching toward' something, which metaphorically became focusing one's mind on it.
Your brain literally 'stretches toward' things when you pay attention—medieval scholars used this same metaphor, thinking attention was like an invisible hand reaching out to grab ideas.
Complete word intelligence in one call. Free tier — 50 lookups/day.