Infinity is a number, or value, which is so huge that it cannot be measured. For instance, the distance to the end of the Universe is called infinity, because if there is an end, it is so far away that it could never be measured.
Sometimes infinity can be the result of a mathematical calculation. For example, the formula for calculating the distance around the outside of a circle is pi times the diameter. Pi is a Greek letter, and it represents a value of approximately 3.14159. Pi can never be fully calculated, because you would finish up with a string of numbers extending to infinity.
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