Intrusive thoughts are obsessive (repeating) unwanted thoughts that someone has that make them feel sad, angry, annoyed, embarrassed, depressed, or even suicidal that led them to feel anxiety or sometimes, even panic attacks. It is normal for everyone to have intrusive thoughts, though some people find it very hard to move on from them because they feel so real.
Intrusive thoughts are most common in obsessive-compulsive disorder (OCD), which is when someone with OCD gets intrusive thoughts that give them extreme anxiety and cause them to do the same ritual over and over again because they feel like bad things will happen if they don't do it. The difference between someone who has OCD and someone who does not is most people without OCD are able to ignore the thoughts, but people with OCD feel like they are holding them back and will not leave them alone. Intrusive thoughts are also associated with depression, body dysmorphic disorder, post-traumatic stress disorder, and attention deficit hyperactivity disorder.