Wikipedia
A Schiff base (named after Hugo Schiff) is a compound with the general structure RC=NR' (R' ≠ H). They can be considered a sub-class of imines, being either secondary ketimines or secondary aldimines depending on their structure. The term is often synonymous with azomethine which refers specifically to secondary aldimines (i.e. R-CH=NR' where R' ≠ H).
A number of special naming systems exist for these compounds. For instance a Schiff base derived from an aniline, where R is a phenyl or a substituted phenyl, can be called an anil, while bis-compounds are often referred to as salen-type compounds.
The term Schiff base is normally applied to these compounds when they are being used as ligands to form coordination complexes with metal ions. Such complexes do occur naturally, for instance in Corrin, but the majority of Schiff bases are artificial and are used to form many important catalysts, such as Jacobsen's catalyst.