Atomic percent or at.% is a measure of concentration of dopant, used in chemistry, physics of solid state lasers and spectroscopy. The meaning of this term strongly depends on the context, host material and dopant. In particular, atomic percent may refer to:

(A) Molar fraction [1] [2], which is ratio <math>~\rm~ \frac{number~oaf~atoms~of~dopant}{number~of~ atoms~ of~ solution}</math>

(B) The molar (or atomic) percentage [3] refers to the ratio
<math>~\rm \frac{number~ of~ atoms~ of~ dopant}{number~of~ atoms~ of~ solution,~ which~ can~ be~ substituted~ with~ the~ dopant}~</math>.

For example, according to the definition (B), 100% doped <math> \rm Yb: Y_3\!~Al_5\!~O_{12}</math> is just <math> \rm Yb_3\!~Al_5\!~O_{12}</math>, not pure Yb, as it would be in the case of definition (A). Definition (B) is widely used in laser physics, and the ambiguity of the term “atomic percent” may cause confusions.

Atomic percent is also used in applications involving atomic isotopes, such as nuclear engineering. In this context, the atomic percent refers to the ratio of the number of atoms of a particular isotope to the total number of atoms in the sample.