taru-adjective, adverb taking the 'to' particle
astonished; shocked; amazed; astounded; aghast
Describes a state of being utterly stunned or dumbfounded, often by unexpected or shocking news. Typically used in the pattern 愕然とする.
その知らせを聞いて、彼は愕然とした。
He was astonished when he heard the news.
突然の解雇に、社員たちは愕然とした表情
The employees looked aghast at the sudden dismissal.
From Chinese 愕然 (èrán), meaning 'stunned' or 'aghast'. The kanji 愕 means 'surprised' or 'startled', and 然 is a suffix forming adjectives/adverbs of state.