Epicene is an adjective used to describe androgyny or a lack of gender distinction. It has historically been used in English to describe effeminacy in males, and less commonly unfeminine behavior in females. In modern usage it has been used as a gender identity, branching out its meanings significantly, but they all carry a general, non-specific “androgynous” tone.
The word comes from Greek: “epi-” (upon/over) + “-cene” (common), from “epíkoinos” which had a “common; sluttish” connotation. It has also been used to describe indeterminate sex.
It is also an important term in linguistics, meaning a word that naturally has only one gender form. It's not used very often in English - but in languages where nouns carry word gender by default, an epicene word is abnormal. Whereas gender-neutral terms of address and occupation (such as "firefighter" rather than "fireman") are common in English, in other languages it can be impossible to refer to people in a gender-neutral manner.
Epicene is also occasionally used to describe bisexual or pansexual attraction.