A phrase used to describe applications and products used to manage and accelerate the flow of data across a wide-area network (WAN). Some of the specific technologies used in WAN optimization include deduplication, data compression techniques, caching, VPN tunneling and other technologies.
An ideal WAN optimization solution will allow you to prioritize traffic, and guarantee a certain amount of available bandwidth for mission critical applications. Complete WAN optimization solutions allow a business to do much more than simply queue the bad traffic. They can block unwanted (in and outbound) traffic, allow it at certain time during the day, give priority to certain hosts, and enforce many other related policies. They will optimize the actual traffic as well, providing lower latency and higher throughput for the most critical applications.
See WAN Optimization 101: Know Your Options on Enterprise Networking Planet to learn more about WAN optimization.