What is an interstitial personality trait? The HEXACO and some other measures of personality describe certain traits as "interstitial". What does it mean for a personality trait to be "interstitial"?
Many personality measures are organised hierarchically where broad traits are defined as the combination of multiple narrow traits. Broad traits are often called domains and narrow traits are often called facets.

Two influential examples are the NEO-PI and the HEXACO models of personality. For example, the NEO-PI has five broad traits and each broad trait is composed of 6 narrow traits. The HEXACO model includes six broad traits and each broad trait is composed of four narrow traits.

However, some measures include narrow traits that do not fit so neatly within a particular hierarchical framework. For example, factor analytic research may indicate that the trait loads substantially on more than one broad trait. 

Interstitial traits are narrow traits that are meaningfully correlated with two or more broad traits.

The terminology comes from the word "interstices" meaning "a small space that lies between things". Thus, such traits occupy a narrow unique space that overlaps with more than one broad trait. 

I'm not sure whether the term "interstitial" could be used to describe traits that are relatively unrelated to any broad trait.