What is the meaning of Nontrivial?

Not trivial.

  1. Not obvious or easy to prove; not self-evident.
  2. Not being of the simplest kind; not being empty or a singleton.

    Not obvious or easy to prove; not self-evident.

    Not being of the simplest kind; not being empty or a singleton.

    Needing significant computing power to solve; intractable.

    Source: wiktionary.org