To subitize is to determine how many objects are present with just a very quick glance. If someone flashes three fingers at you very quickly, you instantly know how many fingers there were. That ability is subitizing. Subitizing is also known as numerosity perception and numerousness perception, and extends to touch and hearing (tactile subitizing and auditory subitizing).

Subitizing without thinking or using a techinque like grouping is known as perceptual subitizing. Humans can generally perceptually subitize up to four objects.

If someone flashes a domino at you, you will grasp both numbers and then add them together to get the total. Subitizing using a techinque such as that is known as conceptual subitizing.

The word "subitize" was coined by Cornelia Coulter from the Latin word subitus (sudden) and the Medieval Latin word subitare (to arrive suddenly). It was introduced in an article in 1949.

Today, subitizing is commonly practiced in elementary schools as a way to build basic math skills and numeracy. The Subitizer website aims to help spread numeracy and build cognitive skills in children and adults through subitizing.