Indefinite Numbers are numbers that are not specifically defined or exact. It's like saying "some" instead of giving a specific number. For example, If someone says "I ate some cookies" they could have eaten 2, 10, or even 100 cookies, we don't know exactly how many. Indefinite Numbers are used when we don't know the exact quantity of something.
Fictitious numbers are basically made-up numbers that we use to help us solve certain types of math problems. Fictitious numbers are usually used in situations where we can't use real numbers. They allow us to come up with an answer even if the answer doesn't actually make sense in the real world.
In summary, Indefinite numbers are used when we don't know exactly how many, while Fictitious numbers are usually made up by mathematicians in order to solve certain types of problems, they may not represent a real quantity.