Overproof rum is rum that is bottled at higher alcohol levels than the standard 40% alcohol by volume. In some bottlings, these rums go as high as 80% ABV. Some overproof rums are marketed with a "151" designation, alluding to their a proof value of 151, which is equivalent to a 75.1% ABV.
These rums are nearly always used in mixed drinks since the alcohol level makes them less palatable than normal proof rums. They are also much more flammable than most spirits and should always be used with caution around open flames.
Some brands of overproof rums include Hamilton Overproof 151, Don Q 151, Goslings 151 Proof Black Seal, Pusser's Select Aged 151, and Rhum J.M. Agricole Blanc 55%.