I have a cyst in my left knee close to the medial meniscus. there is no meniscus tear (confirmed via mri & arthroscopy). could it be a ganglion cyst?

Yes: It could be a ganglion, although meniscal cysts are relatively common, and the difference is probably not important. Speak to your orthopedist if you are in pain to explore your treatment options.

Ganglion cyst : A meniscal cyst is a well-defined cystic lesion located along the peripheral margin of the meniscus, a part of the knee, nearly always associated with horizontal meniscal tears. But some time developed without meniscus tear, I have seen myself several times. Our could a ganglion cyst also. Good luck.

Meniscus cyst: The location you have mentioned is more likely to be a cyst of the meniscus. A ganglion cyst of the joint is possible but knee is a very uncommon location for it.