Computer and network security, or cybersecurity, are critical issues. But merely protecting the systems that hold data about citizens, corporations, and government agencies it is not enough. The infrastructure of networks, routers, domain name servers, and switches that glue these systems together must not fail, or computers will no longer be able to communicate accurately or reliably. Given the magnitude of securing cyberspace, a reflection on what we are trying to do seems in order. Several questions arise, such as what exactly the infrastructure is, what threats it must be secured against, and how protection can be provided on a cost-effective basis. But underlying all these questions is how to define a secure system. What is security? Having it is obviously good; everyone says so. But few people define it exactly, or even nebulously. This column tries to place cybersecurity in perspective, because it is, of course, central to countries, organizations, and even home users now and in the future.