Pavillions are a form of tent used in the SCA. They usually (always?) have at least one open side which is used as a verandah during the day and closed up during the night (or when it rains). Some people put a lot of effort into their pavillions. In day to day usage in the SCA pavilion is used to differentiate a period style tent from a mundane tent such as the ubiquitous dome tent or nylon A frame.
Interestingly, making a period style tent is about as expensive as buying a decent modern tent that is much smaller, and oft-times less practical. Gwynfor Lwyd's 5.5m x 3m Saxon geteld(designed and constructed by House Woodrose) is about $1000 of materials, so the only additional cost is time (and how you cost your time is your business!).
There are larger pavillions which are owned by baronies and so forth (as well as by people who think that bigger is better :-).