Generally attached to another term--like gender-fluid or fluid-sexuality.  It describes an identity that is on a continuum of sexual orientation and/or gender identity.

Recap:  Biological Sex, Gender Identity, Gender Expression, Sexual Orientation
The possibilities are endless...
Biological Sex - your parts - female = vagina, ovaries, XX chromosomes; male = penis, testes, XY chromosomes; intersex = a combination of the two (either internally, externally or both)
Gender Identity - what you think and feel you are regardless of your anatomy and chromosomes
Gender Expression - the ways you act, dress, behave, and interact regardless of your anatomy, chromosomes or gender identity (Someone may be biologically a woman (as far as they know), identify as a woman, and like to dress in mens suits)
Sexual Orientation - who you are attracted to physically, spiritually, and emotionally