A continuous quantity is a quantity that can take any value within one or several intervals. In other words, continuous quantities can be measured to any arbitrarily high degree of precision. Consider a person's age.

The age of a person is changing constantly, depending on how precisely it is measured. Counting only in years, a person could be $5$ or $6$ years old. Counting more precisely, he or she could be $16$ and a half, $10$ years and $3$ months, or even $25$ years and $172$ days old.

If the input of a function is a continuous quantity, the function is said to have a continuous domain. The graph of such a function is typically a curve or a line.

Continuous quantities commonly include length, weight, volume, and difference in time, such as the volume of liquid in a drinking glass or the time that has passed since a person's last haircut.