Atomic weight (symbol: A_r) is a dimensionless physical quantity, the ratio of the average mass of atoms of an element (from a given source) to 1/12 of the mass of an atom of carbon-12 (known as the unified atomic mass unit). The term is usually used, without further qualification, to refer to the standard atomic weights published at regular intervals by the International Union of Pure and Applied Chemistry (IUPAC) and which are intended to be applicable to normal laboratory materials. These standard atomic weights are reprinted in a wide variety of textbooks, commercial catalogues, wallcharts etc., and in the table below.

The term "relative atomic mass" (of the element) may also be used to describe this physical quantity, and is synonymous with it. Indeed the continued use of the term "atomic weight" has attracted considerable controversy since at least the 1960s (see below).

Atomic weights, unlike atomic masses (the masses of individual atoms, not to be confused with relative atomic mass), are not physical constants, but vary from sample to sample of elements that are not mononuclidic elements. This is due to differing isotopic distributions in various samples of non-mononuclidics. Nevertheless, even for elements naturally consisting of two or more nuclides, the atomic weights are sufficiently constant in "normal" samples (those drawn from the environment without special processing) to be of fundamental importance in chemistry.