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The Angstrom (symbol Å) is a non-SI unit of length equal to exactly 10⁻¹⁰ meters (one ten-billionth of a meter) or 0.1 nanometers (nm). While not part of the modern International System of Units (SI), it remains widely used in various scientific fields due to its convenient scale for atomic and molecular dimensions. It provides a direct way to express sizes at the sub-nanometer level without resorting to fractions or powers of ten for nanometers.

The Angstrom unit is named after the Swedish physicist Anders Jonas Ångström (1814–1874), one of the founders of the science of spectroscopy. Ångström used this unit in 1868 to create a chart of the wavelengths of electromagnetic radiation in sunlight, expressing them as multiples of 10⁻¹⁰ meters. This scale proved extremely convenient for expressing atomic radii, chemical bond lengths, and the wavelengths of X-rays. Although the nanometer (nm) and picometer (pm) are the preferred SI units for these scales today, the Angstrom's historical prevalence means it persists in many scientific disciplines, particularly in crystallography, atomic physics, and chemistry literature.

The Angstrom is frequently used to measure dimensions at the atomic scale:

• Atomic Radii: Expressing the size of atoms (e.g., the covalent radius of a carbon atom is about 0.77 Å).
• Chemical Bond Lengths: Measuring the distance between nuclei in molecules (e.g., the O-H bond length in water is about 0.96 Å).
• Crystallography: Describing the dimensions of crystal lattices and the spacing between atomic planes, often determined using X-ray diffraction.
• Spectroscopy: Specifying the wavelengths of X-rays and some ultraviolet light.
• Microscopy: Measuring features in electron microscopy images.
• Integrated Circuits: Sometimes used historically or informally to describe feature sizes in semiconductor manufacturing, although nanometers are now standard.