Here’s how Britannica defines it..

“Naïve art, also spelled naïf art, work of artists in sophisticated societies who lack or reject conventional expertise in the representation or depiction of real objects. Naïve artists are not to be confused with hobbyists, or “Sunday painters,” who paint for fun. The naïve creates with the same passion as the trained artist but without the latter’s formal knowledge of methods.

Naïve works are often extremely detailed, and there is a tendency toward the use of brilliant, saturated colours rather than more subtle mixtures and tones. There is also a characteristic absence of perspective, which creates the illusion that figures are anchored in the space, with the result that figures in naïve paintings are often “floating.” Read more…

Here’s another lovely definition from GINA, Gallery of International Naïve Art

“Naïve art is art that warms the heart and soothes the soul! 

Naïve art is characterized by a refreshing innocence and the charming use of bright colors, child-like perspective and idiosyncratic scale. It portrays simple, easily understandable and often idealized scenes of everyday life. The naïve artist – often self-taught – treats us to a uniquely literal, yet extremely personal and coherent vision of what the world was, is or should be. It offers us, often in painstaking detail, a timeless and optimistic depiction of an ancient story or Biblical tale, an ordinary occurrence or current event, a special ceremony or daily activity. The naïve painting bustles with color and excitement, brims with wry humor and candor, bubbles with unbridled empathy and love.”