Exact is the actual cardinality. How many distinct elements are in the set. HLL can estimate the cardinality by using fraction of the space that would require the store the unique elements instead.
That doesn't match up with the tagline "An improved version of HyperLogLog for the count-distinct problem, approximating the number of distinct elements in a multiset using 20-50% less space than other usual HyperLogLog implementations."
