Hello Gowthamraj
SDN stands for Software Defined Networking, which strictly speaking, is a general umbrella term used to describe all types of networks that are designed using a central controller for the control plane.
SD-WAN is a type of SDN network which applies the principles of SDN on WAN connections. Similarly, SD-LAN is a LAN built around the principles of SDN.
But as we often see, terms are not always used as they were originally meant to be used. Sometimes, the term SDN is used to refer to an SD-LAN, so you must know the context in which the terms are being used.
I hope this has been helpful!
Laz