A septate uterus is where the inside of the uterus is divided by a wall (septum). The septum may extend only part way into the uterus or it may reach as far as the cervix.
My understanding has always been that a septate uterus is a different kettle of fish altogether from PCOS or endometriosis, although I suppose you could have both.

Sending you lots of :hugs: I hope your next ultrasound gives you some comfort ... make you you take a list of questions to ask your Dr when you see them next .. it's often all the unknown factors that make something like this really stressful.