A constructor is a base, suffix or prefix that is used to build new bases and words from existing bases.

Constructor bases are by definition abstract and cannot be used to make words using general modifiers.

A constructor is different from a modifier in that modifiers can be used to create new bases on the fly without official definitions. Constructors cannot be used on the fly and new bases derived from them must be officially recognized. Likewise, a modifier for a particular type of base can be used on any base of that type, but is not necessarily a modifer to other types of bases. On the other hand, constructors can be used on any type of base.

Note that suffixes and prefixes are by definition constructors.

For a list of constructors, see this page.