Pretend
Part of speech: verb
Definition: to claim that or act as if something is different from what it actually is, often incorrectly and knowingly
Example sentence: No psychologist should pretend to understand what he does not understand... Only fools and charlatans know everything and understand nothing.
Make-believe
Part of speech: noun
Definition: The quality or act of pretending; assuming something is true when in fact one knows it is not.