Atom%: absolute abundance of an isotope is the percentage of atoms which occur as the various isotopes (ie. Carbon ¹²C=98.89% ¹³C=1.11%).
Percent Abundance=(# of atoms of given isotope/total # of isotopic atoms) x 100

Atomic number (Z): a term used in chemistry and physics to represent the number of protons found in the nucleus of an atom.

Atomic weight (A): is estimated by adding the number of protons and neutrons of a given nuclide.

Delta (δ): Delta notation is used due to small differences in natural abundance of stable isotopes that otherwise would make data interpretation more cumbersome. For example, δ¹⁵N is a measure of ¹⁵N abundance relative to a reference material. δ is given in units of ‰ (per mil). A δ¹⁵N value of +30‰ means that there are 30 parts-per-thousand or 3% more ¹⁵N in the sample relative to the standard. More information on how δ is calculated can be found in the Theory section.

Depleted (lighter): used to describe the quantity of ¹⁵N or any other isotope relative to another quantity. For example, a substance depleted in ¹⁵N contains relatively less ¹⁵N; furthermore, a substance deplete in ¹⁵N would be relatively enriched in ¹⁴N.

Enriched (lighter): used to describe the quantity of ¹⁵N or any other isotope relative to another quantity. For example, a substance enriched in ¹⁵N contains relatively more ¹⁵N; furthermore, a substance enriched in ¹⁵N would be relatively deplete in ¹⁴N.

Isotope: Isotopes of a chemical element are atoms whose nuclei have the same atomic number, Z, but different atomic mass or mass number, A. The word isotope, meaning at the same place, comes from the fact that isotopes are located at the same place on the periodic table.

Isotope effect: process which results in fractionation. An isotope effect is associated with the majority of all biological, chemical and physical reactions involving isotopes. Fractionation is the physical manifestation of isotope effect.

Fractionation: the unequal partition of isotopes between substrate of a reaction and the products formed during the reaction. More detailed explanation in Fractionation section.

Reference: the comparative method for describing isotope abundance requires that a reference material be used. All measurements are compared to this reference to calculate delta.