Recursion is when something is defined in terms of itself.
Here is the link to the Wikipedia article on this topic.
Note that if something is defined recursively, it may or may not be also expressible without recursion. The Wikipedia article implies, correctly, that the Fibonacci sequence, while defined recursively, can also be expressed without recursion, but stops short of naming any direct (‘closed-form’) representation of the Fibonacci sequence. Binet’s formula expresses the Fibonacci sequence without recursion.
Here is the link to the Wikipedia article that contains a discussion of Binet’s formula.
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