本性
natural instincts; nature; inherent quality
本性难移
It is hard to change one's essential nature (idiom). You can't change who you are.; Can the leopard change his spots? (Jeremiah 13:23)
江山易改,本性难移
it is easier to change mountains and rivers than to alter one's character (idiom); you can't change who you are; Can the leopard change his spots?
第一基本形式
(math.) first fundamental form