Atomic radius is a measure of the size of an atom — typically the mean distance from the nucleus to the outermost electrons. Because an electron cloud has no sharp edge, chemists usually define radius from bonded atoms: for two identical atoms joined by a covalent bond, the covalent radius is half the nucleus-to-nucleus distance.
Other common definitions include van der Waals, ionic, and metallic radii, so the reported value depends on how and in what chemical environment the atom is measured. Across the periodic table, atomic radius generally decreases left to right in a period (higher nuclear charge pulls electrons inward) and increases down a group (more electron shells).