The classic elementary school definition of species is something like “x and y are different species if they cannot produce fertile offspring” which is obviously already incomplete (it causes eg two males of one species to be different species, for example) but could be made a bit more precise.
But the actual definition is much more “a species is a group of reasonably closely related and similar organisms, capable of reproducing fertile offspring given sufficiently many individuals, and the grouping is useful for scientists to talk about”. Obviously that definition isn’t useful for a layperson to decide how to taxonomize things, but I think it’s a useful definition to have when reading about classifications of things into species.
also see “ring species”. there are types of co-habiting gulls who cannot interbreed, but if you follow breeding pairs round the world you can go from one type to the other.
But the actual definition is much more “a species is a group of reasonably closely related and similar organisms, capable of reproducing fertile offspring given sufficiently many individuals, and the grouping is useful for scientists to talk about”. Obviously that definition isn’t useful for a layperson to decide how to taxonomize things, but I think it’s a useful definition to have when reading about classifications of things into species.