Bacterial vaginosis (BV) is an imbalance of the vaginal microbiome, where the protective lactobacilli drop and other bacteria overgrow. It typically causes a thin grey or white discharge with a distinctive fishy odor and usually little or no itching — which helps tell it apart from a yeast infection.
BV is not classed as a sexually transmitted infection, though it's more common with sexual activity and new partners. It usually needs prescription antibiotics from a clinician and commonly recurs; douching makes it worse. See the full bacterial vaginosis guide.