An angstrom or ångström (symbol Å)^[1] is a unit of length that equals 0.1 nanometer (nm). It can be written in scientific notation as 1×10⁻¹⁰ m (normalized notation) or 1 E-10 m (exponential notation) — both mean 1/10,000,000,000 meters. It is sometimes used to express the sizes of atoms, lengths of chemical bonds and visible-light spectra, and dimensions of parts of integrated circuits. For example, a single atom is about two ångströms wide, and a human hair is about a million ångströms thick.

The ångström is named after the Swedish physicist Anders Jonas Ångström (1814–1874), one of the founders of spectroscopy who is known also for studies of astrophysics, flow of heat, the earth's magnetic field, and the aurora borealis.

In 1868, Ångström created a spectrum chart of solar radiation that expresses each wavelength in the spectrum of sunlight in multiples of one ten-millionth of a millimetre, or 1×10⁻¹⁰ metres. This unit of length later became known as the ångström, Å.

Human beings can see light having wavelengths ranging from about 4,000 ångströms (violet) to 7,000 ångströms (deep red), so the ångström was a convenient unit of measurement. Because of its closeness to the scale of atoms and molecules it was also popular in chemistry and crystallography.

Today, the use of the ångström as a unit is less popular than it used to be, and the nanometre (nm) is often used instead (The ångström is officially discouraged by the International Committee for Weights and Measures, and by the American National Standard for Metric Practice.)