Naivete is a lack of knowledge or sophistication.
(noun)An example of a person's naivete is when she falls for an obvious trick or prank.
See naiveté in American Heritage Dictionary 4
or na·ïve·té
nounOrigin:
Origin: French naïveté
Origin: , from Old French naivete, native disposition
Origin: , from naif, artless; see naive
.