To undercount means to count something and end up with a number that's too low because you missed some items or people. When a teacher does a quick headcount of students on a field trip but undercounts by two, she might not realize that some students are still at the museum. When a store manager undercounts the inventory of soccer balls, the computer system thinks there are fewer balls in stock than actually sit on the shelves.

Undercounting often happens by accident: maybe you're counting too quickly, some items are hidden from view, or people move around while you're trying to count them. But undercounting can also be a serious problem when it affects important decisions. If a city undercounts its homeless population, it might not request enough funding for shelters. If scientists undercount an endangered species, they might not realize how urgently it needs protection.

The opposite is overcount, when your total is too high. Census workers try hard to avoid both: they want an accurate count of every person living in the country, not too few and not too many. Undercount can also be used as a noun: “The undercount of students meant the cafeteria didn't prepare enough lunches.”