biologically, it's the woman who gave birth to you.
psychologically, it's the woman who has the most maternal influence over you. could be somebody else apart from your mum.
sociologically, it's who you feel most closely associated to based on the social norms of what a mother is. which means it could be another woman.
Hallmark's definition on the other hand is... they don't care unless you buy a card from them professing/proclaiming your love for your mum.