As an American English native speaker, I did not know this word. Here in the US, a common resource for information is a website called Wikipedia: https://en.wikipedia.org/wiki/Main_Page You can use the search bar in the top right corner to lookup subjects or words. It will tell you the definition and more information regarding the topic. I used this site to find "adnexal". It is a "lump of tissue" in the uterus. So yes, a tumor. It can be harmful or not harmful (benign). For more information: https://en.wikipedia.org/wiki/Adnexal_mass. Hope this is useful for you :]

An adnexal mass is a growth that occurs in or near the uterus, ovaries, fallopian tubes, and the connecting tissues. They're usually benign but are sometimes malignant.