A woman is an adult female human.
In English, the word ‘woman’ refers to a female human who is above a certain age – most people today would say that that age is 18.
Humans are mammals, and all mammalian species’ exhibit binary sex differentiation – that is, every individual of every mammalian species is one of two sexes: male and female. Males have physiologies that are organised around the production of small, mobile gametes. Females have physiologies that are organised around the production of large, immobile gametes.
Note that ‘female’ does not mean ‘having a physiology that is currently producing large, immobile gametes’. The categories of male and female are about which aspects of reproduction the body is organised around.
In humans, sex is determined by genes, rather than environment. Humans with XY chromosomes will typically develop as male, and humans with XX chromosomes will typically develop as female. Note that while genes determine what sex a human will develop as, they do not define what sex is.