taru-adjective, adverb taking the 'to' particle
grave; solemn; stern; authoritative; hard (fact)
Describes a dignified, imposing, or unyielding quality, often of a person's demeanor, a statement, or an established fact. Used in formal or literary contexts.
裁判長は厳然とした態度で判決を読み上げた。
The presiding judge read out the verdict with a solemn demeanor.
それは厳然たる事実であり、誰も否定できない。
That is a hard fact that no one can deny.
厳か emphasizes a quiet, dignified solemnity, often of ceremonies or atmospheres, while 厳然 stresses an imposing, unyielding authority or factuality.
厳しい is a common adjective for 'strict' or 'severe' in everyday contexts; 厳然 is more formal and literary, describing an inherent gravity rather than active strictness.
Compound of 厳 (gen, 'strict, grave') and 然 (zen, a suffix forming adjectives/adverbs meaning 'in the manner of'). The exact historical derivation is uncertain, but the word has been used in formal Japanese to express solemn authority.