Woman is one possible adult gender or sex, typically associated with a certain range of genotypes and phenotypes that are associated with the term female.
Conditions:
It's a spectrum (i.e. not binary, for example intersex people and women that produce more testosterone than some men) so the definition depends on if you're asking about biological sex, legal sex, or gender - they are not necessarily in alignment.
As only individuals can know their own gender, same as goes for things like sexual orientation, that means if someone says they are that gender, the best thing you can do is accept it at face value. This is accepted regardless of the underlying biological sex, because there are few situations that require the biological sex to be known, and it stands in solidarity with people who have gender dysphoria out of respect, as it is not a choice. This is why gender recognition certificates allow people to change their legal sex, because it is a valid and real thing.
Biological sex may appear immutable but ultimately is not immutable, in the same way that all DNA is not immutable, because genes mutate and can be edited, such as with CRISPR. It is also possible that genetics may differ based on other state information, in other words some genetic markers change given hormones applied to them. Hormones and their impact on sex and gender expression are best seen as two ends of a string, or a sympathetic and parasympathetic pathway, in the same way that when trees grow and drop their leaves, it isn't two discrete processes, but in fact one process under given conditions.
Science is not an absolute truth generator either because it can't answer things like are parrots good, and there are actually very few things that are objective - if any at all.
https://www.theguardian.com/world/2024/feb/02/gender-critical-beliefs-under-the-microscope
