from Wiktionary, Creative Commons Attribution/Share-Alike License
- v. to judge (the number of objects in a group) rapidly, accurately and confidently without counting them
- v. to have the ability of judging the number of objects in a group in this way
From Latin subitus ("sudden; unexpected") from the feeling of immediately knowing the number of items present. (Wiktionary)
Specifically, people and animals subitize, or perceive and estimate the number of objects by glance.