SDN = Software Defined Network, these days that's usually an underlay with VXLAN overlays that can be configured and monitored with some kind of GUI and / or programmatically via an API.
SDA = Software Definded Access, Cisco's SDN solution for access networks (ACI is another Cisco SDN but for datacentres)
DNA = Digital Network Architecture - Cisco marketing speak for a raft of things such as their current licensing model and access switch range (including WiFi). Can also refer to DNA Center which is the current Cisco management appliance for access networks (Catalyst + WiFi + Routers) that is also only supported way of configuring Cisco SDA (though you don't need to be doing SDA to use DNA Center it does a lot of other cool things too).
