A "slug," in the context of mass, is a unit of measurement primarily used in the United States customary and British imperial systems. It is not a widely used unit and is considered somewhat archaic. The slug is used to measure mass and is defined as the mass that accelerates at a rate of one foot per second squared (1 ft/s²) when a force of one pound-force (1 lbf) is applied to it. In other words, a slug is the mass that results in an acceleration of 1 ft/s² when a force of 1 lbf is applied to it.
A microgram (µg or mcg) is a metric unit of mass or weight in the International System of Units (SI). It represents one millionth of a gram or 10^-6 grams. The metric system is a decimal-based system of measurement used worldwide, and it provides a convenient way to express both large and small quantities of mass.
To provide context for the size of a microgram:
Micrograms are commonly used in scientific and medical contexts, especially when dealing with substances or quantities that are very small. For example: