Definition: Part-per trillion (ppt) is a unit used to express extremely low concentrations, representing one part of a substance per trillion parts of the mixture. It is often used in environmental science, toxicology, and chemistry to quantify trace amounts of pollutants or contaminants.

History/origin: The concept of 'parts per' notation originated from the need to express very small concentrations or proportions. 'Part per trillion' is a logical extension of 'parts per million' (ppm) and 'parts per billion' (ppb), becoming relevant with advancements in analytical techniques that allowed for the detection of increasingly smaller quantities.

Current use: PPT is commonly used to measure trace contaminants in water, air, and soil. It is also used in pharmaceutical analysis to quantify impurities, and in forensic science to detect trace amounts of substances in samples. Regulatory agencies often use ppt to set permissible levels for pollutants.