expression, godan verb (-ru)
break the seal; open a sealed item
Used for physically breaking a seal on a letter, envelope, bottle, or container. Also used figuratively for starting something new or opening up a situation.
手紙の封を切る。
I break the seal on the letter.
新しいウイスキーの封を切った。
I opened a new bottle of whiskey.
開封する is a more formal, often written term for opening a sealed item, while 封を切る is a common, everyday expression.
Compound of 封 (seal) + を (object marker) + 切る (cut). Literally 'cut the seal'.