The sorites paradox is a philosophical problem dating from the time of (at least) Ancient Greece.
It relates to the problem of defining groups - specifically large groups. It has implications for the definitions of 'vagueness' 'fuzziness' 'bluriness' etc etc. Which in turn have implications for mathematics, modern computing systems (search algorithms etc etc )
An example often used is the 'Paradox of the Heap'
A pile of sand with thousands of grains can be called a 'heap'. If one grain is removed, it's still a heap. If, however, this step is repeated enough times, it will no longer be considered a 'heap'
Another example is the 'Bald Man'. If a man has a full set of hair, and one hair is removed, he isn't bald. but if the process is repeated, he will at some stage be classified as bald. Where is the boundary?
Further reading : Stanford Encyclopedia of Philosophy