Software-Defined Network (https://www.examguides.com/CCNA/cisco-ccna-15.htm) is a network architecture that allows a network to intelligently and centrally control or 'program' software using applications. It enables operators to manage the entire network consistently and comprehensively, regardless of the underlying network technology.
SDN allows the programming of network behavior in a centrally controlled manner through software applications using open APIs. By opening traditionally closed network platforms and implementing a simple SDN control layer, operators can consistently manage the entire network and its devices regardless of the complexity of the underlying network technology.