Question

What is well defined collection of an object with an example

Answer 1

Here “well defined” means that any given object in the world at large (abstract or concrete) is either an element of S or it isn't.

Example: “The collection of young people that are residents of Minnesota” is not a set. What does it mean be "young"? To someone that is 80 years old, a person that is 40 is young, but a 20 year old person may not consider 40 to be young. The word “young” is an ambiguous term. Hence, the collection is not well-defined.

Example: “The collection of residents of Minnesota between the ages of 18 and 25 years old inclusive” is a well-defined set since it is fairly clear who would belong to this collection.