Fetching details from the dictionary...

stick-to-itiveness; perseverance; persistence
radical (esp. one that tends to ionize easily)
root
indriya (faculty of the body having a specific function, i.e. the sensory organs)
彼は根がとても優しい人です。
He is a kind person at heart.
あの人は根があるから最後まで頑張る。
He has persistence, so he will stick it out.
試験前に根を詰めて一生懸命勉強しました。
I studied hard without rest.