also: せきぜん
taru-adjective, adverb taking the 'to' particle
fully satisfied; cleared of doubt; convinced
Almost always used in the negative form 釈然としない to express lingering dissatisfaction or doubt after an explanation or apology. The positive form is rare.
See also: 釈然とする
彼の説明を聞いても、どうも釈然としない。
Even after hearing his explanation, I'm not fully satisfied.
謝
There was an apology, but a feeling of not being fully convinced remained.
納得 is a common noun/suru-verb for understanding and accepting something logically. 釈然 is more literary and almost exclusively used in the negative 釈然としない to describe an emotional sense of unresolved doubt.
すっきり is a casual adverb for feeling refreshed or clear-headed. 釈然としない is more formal and specifically refers to mental dissatisfaction with an explanation or situation.
The kanji 釈 means 'explain' or 'release', and 然 is a suffix meaning 'state' or 'condition'. Together they suggest a state of having been explained or resolved. The exact historical derivation is uncertain, but the word is used in classical and modern Japanese to describe a feeling of clarity or satisfaction.