In mathematics, "uncountable" refers to a type of infinity that is larger than the infinity of natural numbers. While you can list natural numbers like 1, 2, 3, uncountable sets, such as the set of all real numbers, cannot be listed in this way. This means that there are more real numbers than there are whole numbers, even though both sets are infinite. A common example of an uncountable set is the set of all points on a line segment. No matter how you try to list them, there will always be more points than you can count. This concept helps us understand different sizes of infinity and is a key idea in set theory and mathematics.