Introduction. A Bartholin's cyst, also called a Bartholin's duct cyst, is a small fluid-filled sac just inside the opening of a woman's vagina. You may feel a soft, painless lump.This doesn't usually cause any problems.ntroduction. A Bartholin's cyst, also called a Bartholin's duct cyst, is a small fluid-filled sac just inside the opening of a woman's vagina. You may feel a soft, painless lump.