Uptimeis a computer industry term for the time during which a computer is operational. Downtime is the time when it isn't operational. Uptime is sometimes measured in terms of a percentile. For example, one standard for uptime that is sometimes discussed is a goal called five 9s - that is, a computer that is operational 99.999 percent of the time.
In general, uptime is considered the default where uptime is contrasted with downtime. Downtime often requires specific explanation or description, such as downtime for maintenance, downtime as a result of malfunction/error or downtime resulting from a crisis.