angstrom

Short definition from Wiktionary

Noun

1. 1
   A unit of length equal to 10⁻¹⁰ meters (that is, one ten-billionth of a meter), approximately the size of an atom, and denoted by the symbol Å, used especially to measure the wavelength of electromagnetic radiation or distances between atoms.
2. 2
   Alternative letter-case form of ang­strom.