An enclitic is a clitic that can occur at the end of a free form but not at the beginning.
Enclitics are often defined with respect to "phonological attachment" or phonological units (as in the definition of the SIL Glossary: "An enclitic is a clitic that is phonologically joined at the end of a preceding word to form a single unit"). But the definition in terms of free form is easier to apply, because the nature of phonological units is often unclear.