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An amidine is a nitrogen analogue of an oxoacid derivative. In ordinary organic-chemistry usage it most often means a carboxamidine, whose central carbon is bonded to two nitrogens, conventionally represented by $\mathrm{R}-\mathrm{C}(=\mathrm{N}{\mathrm{R}}_1)-\mathrm{N}({\mathrm{R}}_2){\mathrm{R}}_3$. The IUPAC class is broader than only carbon-centred examples, so the intended structural subclass should be stated.^[ref]

A simple unsubstituted-nitrogen pattern is $\mathrm{R}-\mathrm{C}(=\mathrm{NH})-\mathrm{N}{\mathrm{H}}_2$. Choosing $\mathrm{R}=\mathrm{C}{\mathrm{H}}_3$ gives $\mathrm{C}{\mathrm{H}}_3-\mathrm{C}(=\mathrm{NH})-\mathrm{N}{\mathrm{H}}_2$ and an atom count of ${\mathrm{C}}_2{\mathrm{H}}_6{\mathrm{N}}_2$. Compare an amide, $\mathrm{R}-\mathrm{C}(=\mathrm{O})-\mathrm{N}{\mathrm{H}}_2$: replacing the doubly bonded oxygen with an imino nitrogen changes the characteristic group. Similar spelling does not make the two classes chemically identical.

Imidoesters react with amines to give amidines. This conversion connects a functional-group definition with experimental protein cross-linking, where amino groups on a protein can be modified by an appropriate reagent.^[ref] The structural pattern alone does not specify one reaction rate or one acid–base constant for every substituted amidine.

A carboxamidine is a carbon-centred amidine, with the N-unsubstituted pattern $\mathrm{R}-\mathrm{C}(=\mathrm{NH})-\mathrm{N}{\mathrm{H}}_2$. More generally, substituents may replace hydrogens on either nitrogen. Its defining carbon is attached to an imino nitrogen and an amino nitrogen; an amide instead has a doubly bonded oxygen at the corresponding position.^[ref]

For an atom-count example, $\mathrm{C}{\mathrm{H}}_3-\mathrm{C}(=\mathrm{NH})-\mathrm{N}{\mathrm{H}}_2$ has molecular formula ${\mathrm{C}}_2{\mathrm{H}}_6{\mathrm{N}}_2$. Replacing one amino hydrogen by $\mathrm{C}{\mathrm{H}}_3$ gives $\mathrm{C}{\mathrm{H}}_3-\mathrm{C}(=\mathrm{NH})-\mathrm{NH}-\mathrm{C}{\mathrm{H}}_3$, with molecular formula ${\mathrm{C}}_3{\mathrm{H}}_8{\mathrm{N}}_2$. The change adds $\mathrm{C}{\mathrm{H}}_2$ overall because the new methyl group replaces a hydrogen. This bookkeeping illustrates N-substitution without treating a structural drawing as a measured reaction yield.

Class names and preferred naming suffixes should also be distinguished. The IUPAC recommendations retain the generic class description but use the suffixes -imidamide and -carboximidamide in the specified nomenclature, and distinguish the amino and imino positions by N and N′. Calling a substance a carboxamidine therefore does not prescribe the suffix in its preferred systematic name.^[ref]