Picogram is a unit of measurement equal to one trillionth of a gram. There are several synonyms that can be used interchangeably with the word picogram. These include trillionth of a gram, femtogram, and microgram. The term femtogram is derived from the metric prefix "femto" which means one quadrillionth or 10^-15, while microgram is equivalent to one millionth of a gram or 10^-6. These synonyms are commonly used in scientific research and medical fields where precise measurement of elements and substances is critical. Whichever term is used, the main purpose is to accurately convey a small quantity of mass or weight.