  We report the optical imaging of a single atom with nanometer resolution
using an adaptive optical alignment technique that is applicable to general
optical microscopy. By decomposing the image of a single laser-cooled atom, we
identify and correct optical aberrations in the system and realize an atomic
position sensitivity of $\approx$ 0.5 nm/$\sqrt{\text{Hz}}$ with a minimum
uncertainty of 1.7 nm, allowing the direct imaging of atomic motion. This is
the highest position sensitivity ever measured for an isolated atom, and opens
up the possibility of performing out-of-focus 3D particle tracking, imaging of
atoms in 3D optical lattices or sensing forces at the yoctonewton (10$^{-24}$
N) scale.
