Off the top of my head, "A collection of organisms which can interbreed with each other and produce fertile offspring", for sexual organisms, and "what humans decide is a species" for asexual organisms. Would an expert be able to do better? The word seems too old and the concept to vague to have a tight definition.
"Species" is not a clean concept in a world with viruses, clines, and ring species.
More precisely, "species" is a map marker made by someone who likes discrete, mostly tree-like maps (legacy of Aristotle?)
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