El factor de disponibilidad de un equipo o sistema es una medida que nos indica cuánto tiempo está ese equipo o sistema operativo respecto de la duración total durante la que se hubiese deseado que funcionase. Típicamente se expresa en porcentaje. No debe ser confundido con la rapidez de respuesta.

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In reliability theory and reliability engineering, the term availability has the following meanings: \n* The degree to which a system, subsystem or equipment is in a specified operable and committable state at the start of a mission, when the mission is called for at an unknown, i.e. a random, time. Simply put, availability is the proportion of time a system is in a functioning condition. This is often described as a mission capable rate. Mathematically, this is expressed as 100% minus unavailability. \n* The ratio of (a) the total time a functional unit is capable of being used during a given interval to (b) the length of the interval. For example, a unit that is capable of being used 100 hours per week (168 hours) would have an availability of 100/168. However, typical availability values are specified in decimal (such as 0.9998). In high availability applications, a metric known as nines, corresponding to the number of nines following the decimal point, is used. With this convention, "five nines" equals 0.99999 (or 99.999%) availability.

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