STP stands for Standard Temperature and Pressure, which is a set of conditions often used as a reference point in chemistry for measuring gases. Specifically, STP is defined as a temperature of 0 degrees Celsius (273.15 K) and a pressure of 1 atmosphere (101.3 kPa). Understanding STP is essential in reaction stoichiometry because it allows for consistent comparisons of gas volumes and supports calculations involving the ideal gas law.