Femto is a prefix used to describe a very small unit, one quadrillionth of a unit. Antonyms for femto would be prefixes used to describe larger units. Examples of these prefixes include "micro," meaning one millionth of a unit, "milli," meaning one thousandth of a unit, "centi," meaning one hundredth of a unit, "deca," meaning ten times a unit, "hecto," meaning one hundred times a unit, and "kilo," meaning one thousand times a unit. These prefixes are commonly used in measurements, such as in metric units like kilometer, milligram, and hectoliter.