downtime [outage, failure, crash, opposite to uptime] means a period when a website is unavailable.
In terms of website monitoring services, unavailability is confirmed when multiple monitors confirm one of the following:
a) the monitored URL returns an error code (a number different than "200 OK")
b) the monitored server doesn't answer within 5 seconds (timeout)
c) the monitored page doesn't contain the closing tag "