expression, i-adjective
single-minded; completely absorbed; devoted entirely to one thing
Describes a state of being so focused on one thing that no other thoughts intrude. Often used in the pattern 〜に余念がない.
彼は研究に余念がない。
He is completely absorbed in his research.
子供たちは遊びに余念がなかった。
The children were completely engrossed in their play.
余念がない is a phrase describing a state of total concentration, while 一心不乱 is a more formal, often adverbial noun meaning 'wholeheartedly' or 'with undivided attention'.
夢中 means 'absorbed' or 'crazy about' and can be used for hobbies or infatuation, whereas 余念がない emphasizes the absence of other thoughts during focused work or activity.
From 余念 (よねん, 'other thoughts') + がない ('there is no'), literally 'there are no other thoughts'. The expression is straightforward and its meaning is transparent from its components.