femtogram

This term is a specialized scientific unit used almost exclusively in high-precision fields such as molecular biology, genomics, and nanotechnology. It describes masses so minute that they are invisible to the naked eye and require advanced instrumentation, such as atomic force microscopy or ultra-sensitive mass spectrometry, to detect.

Because it is a standard unit of measurement, it follows the typical pattern of metric units. While it can be used in the plural to describe a quantity of mass, it is never used in a casual or metaphorical sense to describe something small; it remains strictly a technical term for quantitative measurement.