The availability of a website is measured by the percentage of a year in which the website is publicly accessible and reachable via the Internet. This is different from measuring the uptime of a system. Uptime refers to the system itself being online. Uptime does not take into account being able to reach it as in the event of a network outage. A hosting provider's Service Level Agreement (SLA) may include a certain amount of scheduled downtime per year in order to perform maintenance on the systems. This scheduled downtime is often excluded from the SLA timeframe, and needs to be subtracted from the Total Time when availability is calculated. Depending on the wording of an SLA, if the availability of a system drops below that in the signed SLA, a hosting provider often will provide a partial refund for time lost. How downtime is determined changes from provider to provider, therefore reading the SLA is imperative. Not all providers release uptime statistics. Most hosting providers will guarantee at least 99.9% uptime which will allow for 43m of downtime per month, or 8h 45m of downtime per year.
Shared hosting is web hosting in which the provider houses multiple sites on a single server. For example, Site A shares the same server with Site B, Site C, Site D, and Site E. The upside is that the multiple sites share the server cost, so shared web hosting is generally very inexpensive. In fact, you can find an option for less than $10 per month.
^ March 16, 1992 memo from Mariam Leder, NSF Assistant General Counsel to Steven Wolff, Division Director, NSF DNCRI (included at page 128 of Management of NSFNET, a transcript of the March 12, 1992 hearing before the Subcommittee on Science of the Committee on Science, Space, and Technology, U.S. House of Representatives, One Hundred Second Congress, Second Session, Hon. Rick Boucher, subcommittee chairman, presiding)