Describes something so impressive, beautiful, or exciting that it takes one's breath away. Often used for scenery, performances, or moments of great beauty or intensity.
The scenery was breathtaking.
It's a breathtaking beauty.
Standard kanji spelling using 呑む, which is common in this expression.
Variant using 飲む; both spellings are used, but 呑む is more typical for this figurative sense.
From the verb phrase 息を呑む (to hold one's breath, to gasp) + よう (appearing, seeming). The expression figuratively describes something so striking that it makes one catch one's breath.