taru-adjective, adverb taking the 'to' particle
cool; indifferent; nonchalant; calm
Formal literary term describing someone who remains composed and unruffled, often in a situation where others might react emotionally. Used with と as an adverb or in the たる form as an adjective.
彼は非難されても恬然としていた。
He remained cool and indifferent even when criticized.
恬然たる態度で交渉に臨んだ。
He approached the negotiation with a calm, nonchalant attitude.