Zero Touch Provisioning covers the process of getting devices under management without manual configuration. For example if you connect a factory default Access Point or switch to a network, that it will be automatically discovered and added to the management platfom (Airwave/Central in Aruba) which then can push the configuration. This eliminating the requirement to sendian engineer on-site with a console cable to enter initial configuration to make a device reachable so it can be managed and further configured. Or the requirement to unpack new hardware and 'stage' it with initial configuration before you ship it to the destination location.
Software Defined Network is a somewhat ambigeous term that ranges from pushing configuration to a device (which is more automation) to having user and network flows being controlled by a central entity. One of the important aspects is that it is dynamic and flows/network behavior can change and adapt to the circumstances. Example: user connects to a wired switch port, on the fly the user is authenticated and status is derived, and a policy is pushed to allow certain traffic or tunnel traffic to a central location.
SDN will come after ZTP (or manual provision).