Obsessions are unwanted, intrusive, repetitive sometimes constant thoughts which are difficult or impossible to control.
These thoughts can produce tremendous anxiety.
The thoughts can range from inoffensive to very offensive or intrusive.
The frequency can range from now-and-then to almost continuous.
Examples of obsessive thoughts are:
I think I left the stove on,
I think I left the door unlocked,
or may include disturbing thoughts of a sexual or violent nature.
For more examples of obsessions see the Tic List.