An atmospheric gas or gas combination whose abundance and environmental context can support a biological interpretation. Detectability, a plausible production flux and exclusion of significant abiotic alternatives matter together. Modern-Earth molecular oxygen and nitrous oxide have predominantly biological sources, but an oxygen biosignature false positive illustrates why extrapolation to another planet requires context.
Nonbiological molecular oxygen can accumulate when photochemistry and planetary redox sinks allow it, for example after water photolysis and hydrogen loss. Ozone produced from that oxygen is not independent proof of a biological source. The stellar spectrum, water inventory, atmospheric escape and other gases constrain these interpretations.
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