any thought that is unwanted is inteuive. they don’t always have to be pop ups, you can just ruminate on that one theme for hours & have reoccurring thoughts. this would be OCD, obsessively thinking of the fear, doing compulsions like searching for answers, and finding reassurance, just to restart the cycle. normal people have intrusive thoughts all the time, but what makes people with OCD different is we obsess over the intrusive thoughts & affect the quality of our lives. :) i hope this helps.
Like hope said, any thought that is unwanted or a thought that you don’t like that is in your head is intrusive thoughts.some are worse than others.