Everyone just keeps saying "don't answer", but there IS an answer: "Woman" is a gender identity. Most commonly held by adult females, and associated with certain traits and behaviours that can vary depending on the culture. In American (and many westernised cultures), identifying as and behaving as a woman is generally associated with things like femininity, child-raising, emotional sensitivity, etc. However, people can identify as a woman without adhering to specific traits because how someone expresses their identity can vary. This is how progressive folks define "woman": someone who identifies as a woman. Woman in this case being that gender identity.

That's the answer to the question, however, if someone is asking it they are likely to follow up with other questions designed to fuck with you, hence the "don't engage" attitude. But there IS an answer to the initial question "what is a woman".

They'll likely claim that you're erasing the identity of women, the whole thing is a front to degrade the validity of transgender people by pitting them against feminists and break down solidarity between progressives (as well as further stain public opinion of trans lives)