This is the 3228th most frequent Japanese word.
天才
Genius.
In this sentence, '天才' (tensai) is used to describe someone's exceptional ability in the specific domain of mathematics, emphasizing intellectual giftedness.
彼は数学の天才です。
He is a genius in mathematics.
Here, '天才的' (tensaiteki) derived from '天才' is utilized as an adjectival form to signify the extraordinary and brilliant nature of the child's artistic work.
あの子の描いた絵は天才的ですね。
The painting drawn by that child is genius-like.
In this context, '天才' is used metaphorically to express extreme proficiency in playing the piano, conveying the person's outstanding talent.