An angstrom (Å)
is a unit of length, essentially 10-10
meters long, i.e., 1/10 of a nanometer (nm).
It is not an official SI unit (nor is it a CGS unit),
but was defined for use in some fields of physical science.
Both angstrom and nanometer are convenient for use in
spectrography and atomic physics, such as
the study of electromagnetic radiation (EMR) interaction with atoms.
They are convenient for EMR wavelengths in the range of
near infrared, visible light, and ultraviolet.