Availability is a key measure in Service Level Agreements (SLAs) between service providers and their customers as well as their vendors and suppliers. Metrics for estimating IP packet layer availability [Y.1540] and service level availability [ATIS-0100025] have received considerable attention in various standards bodies. In order for service providers, vendors or customers of the network to develop an SLA that includes the availability metric, the definition of availability must be agreed upon and a method for estimation developed. This Technical Report (TR) will describe the components of the definition of "Availability" that should be present. The purpose is to stimulate consistency in the manner which availability is defined in various standards documents.