na-adjective, noun
wholeheartedly; single-mindedly; with intense concentration
Describes doing something with complete focus and dedication, without distraction. Often used with verbs like 取り組む, 勉強する, 祈る.
彼は一心不乱に研究に取り組んでいる。
He is wholeheartedly engaged in his research.
試験前、彼女は一心不乱に勉強した。
Before the exam, she studied with intense concentration.