expression, noun which may take the genitive case particle 'no'
breathtaking; stunning; awe-inspiring
Used as a prenominal modifier (～の) to describe something so beautiful, impressive, or shocking that it takes one's breath away.
息を呑むほど美しい景色だった。
It was a breathtakingly beautiful view.
彼女の演技は息を呑むほど素晴らしかった。
Her performance was stunningly wonderful.
息を呑む is the verb phrase 'to hold one's breath' or 'to be breathless', while 息を呑むほど is the adverbial/adjectival form meaning 'to the extent of being breathless'.
From 息 (breath) + を + 呑む (to swallow, hold) + ほど (extent). Literally 'to the extent of swallowing one's breath', used to describe something so striking it makes one hold their breath.