noun
Refers to the concept of relativity, especially in physics (e.g., Einstein's theory) or in a general sense of things being relative.
アインシュタインの相対性理論は有名だ。
Einstein's theory of relativity is famous.
価値の相対性について議論した。
We discussed the relativity of values.
相対的 is a na-adjective meaning 'relative', used to describe something as being relative, whereas 相対性 is the noun form for the concept of relativity.
Compound of 相対 (sōtai, 'relative') + 性 (-sei, '-ness, -ity'), forming the abstract noun 'relativity'.