I would say that adverse means harmful or unwanted. The definition "having a crossing effect" seems a little strange to me. Can you post the link to where you found that definition?
Here is an example of the use of adverse:
I took the medication but it had an adverse effect on me = I took the medication but instead of making me better, it prompted a bad reaction from my body (a side-effect of some sort).
Don't go sailing on Sunday. They're warning of adverse weather conditions = Don't take the boat on the sea on Sunday. The forecasters are saying that the weather will be bad.
Student or Learner