What does closed in math mean?

In mathematics, "closed" can refer to a set that contains all its limit points, meaning that if a sequence within the set converges to a point, that point is also included in the set. For example, the interval [a, b] is closed because it includes its endpoints a and b. Additionally, a function is said to be closed if the image of a closed set under that function is also closed. This concept is important in topology and analysis.