Verisimilitude, which comes from Latin verum—truth—and similis—similar, is “truth similarity," or the effect of being true-to-life, generally applied to art in some form. What's interesting is determining what true-to life is. What is "life" (reality) for the particular work in question? According to Plato & Aristotle, for a work of art to have significance, it must be grounded in reality. For literature, an author must know what the truth is—the truth of the setting, characters, plot, and themes—and use his tools (dialog, narrative, imagery, characterization, etc.) to create a literary “similarity” to that truth. If he accomplishes that, he has achieved verisimilitude.
Why is that such a big deal? Why do English teachers spend so much time harping on the language in works of lit? Because verisimilitude may be the pinnacle of achievement in literature. What’s exciting about verisimilitude is that it’s different for every single work out there (and, I think, for every reader, too). For Twain, it might be the life of an uneducated backwoods boy. For Nora Roberts, it might be the developing relationship of between the hero and heroine. A novel set in the Russian Revolution will have characters named Ivan or Dmitri, not Bill or Sam. A story about a boy who learns he's a wizard will be filled with moments of discovery and learning of magic, not mundane tasks like walking the dog.
Verisimilitude is—I suspect—a very subtle thing, and easily passes without note, because it’s woven so tightly into the fiber of the story that it should be unnoticeable. Until we look deeper at a piece, it’s hard to see whether it’s made it. It’s possible that verisimilitude might depend on the reader and her interpretation. It’s achieved in the interplay of the artist, the artwork, and the reader. How amazing, as a reader, to be part of a work of art by appreciating it.
What fascinates me about verisimilitude in literature is, of course, the use of language to achieve it. That's a big part of what I got out of my thesis; the roll of language in lit. I suspect I'll be blogging more on this in the future!