A villanella was a light-hearted, generally simple type of Neapolitan madrigal popular during the 16th century. It was similar to the Italian frottola and English ayre in that it usually consisted of a number of verses sung all to the same tune and, like them, was less contrapuntal than the true madrigal or chanson of the time.
References:
“Oxford Companion to Music”
“The Grove Concise Dictionary of Music”