Experts in both biology and gender regard "woman" as too variable a marker to be encompassed by one simple definition.
A woman is colloquially understood as an adult human with female biology. However, some individuals have a genotype (internal biology) that conflicts with their phenotype (external physical characteristics). For example, 1 in 20,000 individuals have androgen insensitivity syndrome, possessing XY chromosomes and male internal reproductive organs (testes) while exhibiting a female phenotype (vagina, breasts). These individuals typically live as women despite their internal male biology.
In addition to biological ambiguities, there is a medically recognized difference between biological sex and psychological gender. Some individuals report feeling like a woman and exhibit feminine traits despite being considered male at birth, and vice versa. Numerous studies have found that allowing transgender individuals to live as their desired gender lowers their risk of depression and suicide.