Question

difference between infinity and not defined​

Answer 1

Infinity is nothing but greater than every real number. It's a concept, not a number, though there are "numbers" associated to collections of infinite objects, but that's a huge area of maths, not suitable for an answer, maybe a book. "Not defined" means, well that what you have cannot be defined.

Answer 2

Infinity means any no. which can't be imagined

whereas not defined refers to a no. which is not well defined but it can be imagined