“A community, or a number of related communities whose distinctive morphologyical characters are, in the opinion of a competent systematist, sufficiently definite to entitle it, or them to a specific name.”
-Charles Tate Regan, 1926
There are of course other definitions of a species but they all pretty much epistemologically boil down to the one above. One might protest that another classic definition i.e. "a species is known when two of it's members can produce fertile offspring" is better. But even this definition is not really definite, look up hybrids, and in any case is not always adhered to by taxonomists.
Genetics won't help you either. At some point, if the term species is to have any meaning at all, somebody, and we have given these people the name "taxonomists," must make a judgement call as to what amount of variation in morphology or genetic code, as Regan puts its is "sufficiently definite to entitle it" the rank of species.
In this sense new species are never actually "discovered" as is commonly asserted, but rather created by taxonomists according to arbitrary, no matter how rationale, rules.
Next time you read in Nature about the discovery of a new species of insect in the Amazon, know that it is hogwash. The species was created, not found. Admittedly thinking this way feels weird. We are so used to knowing that a species is something definite and discover-able, like gravity, that to think of it as a construct is actually cognitively difficult.
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