Knifepoint is the use of a knife to threaten someone — to make them give you money, get in a car, keep quiet. You almost always meet it in the phrase at knifepoint: 'she was robbed at knifepoint.' You will also see knifepoint robbery in the news. It names the threat, not a type of knife, and not the sharp tip as a tool. Formal enough for crime reports; grimly ordinary there. Though actually people do not say 'a knifepoint' the way they say 'a knife'; the word lives inside that at-phrase.