An angstrom or ångström (Å) is a non-SI unit of length equal to 10−10 metres, 0.1 nanometres or 100 picometres.
The angstrom is named after the Swedish physicist Anders Jonas Ångström, one of the founders of spectroscopy. It is often used for expressing the sizes of atoms, whose radii are between 0.25 and 3 Å, and the lengths of chemical bonds, which are typically between 1 and 2 Å. For a list of objects of size 1 to 10 Å, see 1 E-10 m.
This is a non-SI unit, and therefore its use is officially discouraged. It is listed in Table 8 of the SI brochure ("Other non-SI units currently accepted for use with the International System"). BIPM explains "Table 8 lists some other non-SI units which are currently accepted for use with the SI to satisfy the needs of commercial, legal and specialized scientific interests. These units should be defined in relation to the SI in every document in which they are used. Their use is not encouraged."
Nanometres or picometres can easily be used instead. However, despite its official deprecation, many scientists continue to use it. They claim it is a more convenient unit, corresponding more closely to the size of the items being discussed, e.g. atoms, grains of interstellar dust, optical wavelengths, etc.
For historical reasons, astronomers prefer to use the angstrom when working with visible-light spectra.
Unicode includes the "angstrom sign" at U+212B (in your browser it looks like ); still the standard states that the "preferred representation is 00C5" — i.e. it is better to use "Latin capital letter with ring above" (Å). The "angstrom sign" is canonically decomposed into U+00C5 — i.e. it is a singleton.
See also