A clitic is a bound morph that is not a root or an affix.
The term "clitic" is used very inconsistently in the literature, but typically in contrast to "affix", and very often in contrast to "free form" (there is thus widespread agreement that clitics are bound forms). However, many authors say that affixes are somehow "more bound" than clitics, and/or that clitics are somehow "phonologically deficient" (and thus in need of a host). These statements have never been made fully precise. In the definition given here, there is no reference to phonological properties, and all that counts is indiscriminacy (or "promiscuity", i.e. lack of word-class selectivity).