A measurement of the concentration of dissolved solids in a wort that is used to quantify the concentration of extract as a percentage by weight.
Each 1 °P generates about 0.4% alcohol by volume (ABV). Thus, a 15 °P wort will produce approximately 6% ABV.
The Plato gravity scale can be approximated to the specific gravity scale (used by brewers in the UK and others using British brewing traditions) by multiplying the °P by 4 to give the degrees of gravity. Therefore, a 15 °P wort is approximately 60 degrees of gravity.