Cyster is a slang term for a woman with polycystic ovary syndrome (PCOS). It is a play on the words "cyst" and "sister" and is meant to promote a sisterhood of women for support.
FertilitySmarts explains Cyster
Several online groups exist to give support to women with PCOS and their struggles with fertility and pregnancy. Several website groups, such as cysters.co.uk, soulcysters.com, and soulcysters.net exist to give support to women with polycystic ovary syndrome (PCOS). Their focus is on all aspects of living with PCOS but also touches a great deal on the struggle with fertility and pregnancy.
These online groups are designed to support women in a non-judgmental environment and empower them to overcome the shame that may erroneously be associated with gynecological problems. Often, women do not feel comfortable discussing their issues with their doctors or nurses, and these forums allow women to attain useful information about their conditions and talk with other women who have gone through the same struggles.