Individual with which one shares a common ancestor.
Note that relatives vary in terms of how far one must go back in time to find that common ancestor. For close relatives one need not go back in time very far whereas for distant relatives one instead must go back somewhat in time.
Note that the concept of relative does not change when considering relationships that exist within species versus relationships that exist between species. The members of an ancestral species, as members of a common gene pool themselves, can just as reasonably be described as relatives as too are all of the members of one's own species, as indeed are all of the members of one's nuclear family. It's just a matter of specifying distance, where ancestral species are more distant relatives than members of one's own species versus one's brothers and sisters.
One can also consider relatives, either within or between species, as relations by blood. Intimately tied up with these ideas also is that of vertical gene transfer, a.k.a., vertical inheritance.
A great complication on the idea of who is related to whom is horizontal gene transfer, which results in individuals sharing some alleles or genes but not others. Indeed, ultimately an individual shares more alleles with closer relatives than they do with more distant relatives, that is, there exists greater homology among closer relatives, while horizontal gene transfer can create homologies without an equivalent close sharing of common ancestors.
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