1 : of equipment, the number of hours producing, plus standby time, divided by total available hours; expressed as a percentage: 168 hours - (facilities downtime + equipment downtime + engineering time + setup and test time)/168 hours X 100. [SEMATECH] 2 : of equipment, the probability that the equipment will be in a condition to perform its intended function when required. [SEMI E10-92] 3 : of computer systems, the amount of time a computer system, communications network, or database system is ready to support normal activity. [SEMATECH]