Something that merely seems to be true or real. Many writers or filmmakers try for some kind of verisimilitude in their stories, to make them believable. Including a great many details in a novel, for example, adds to its verisimilitude.
E.g. She was experiencing a nightmare of the most extraordinary verisimilitude.
For some general tips on how you can improve your English, take a look at our previous blog post, that will tell you How to develop good English communication skills.