A Convention is a gathering of people meeting for the purpose of discussing or enjoying some aspect of Science Fiction or Fantasy. Sometimes a convention will have a theme or focus on one or a few genres of SF rather than attempt to have programming for all of them
Well established conventions will typically have an ongoing name or nickname for themselves (e.g. World Science Fiction Convention / WorldCon, Westercon, Noreascon, ComicCon, etc.) that may reflect their location or their intended scope. They also will often be numbered as successive similar conventions are held. Naming can be simple and straightforward (e.g. LA Con I, LA Con II, LA Con III, etc.) or complex and whimsical (e.g. Gallifrey One, Gallifrey One: the Sequel, Gallifrey One in 3D, Gallifrey One Goes Fourth, etc.).
Size may vary from a few hundred attendees that might visit a small local gaming convention to over 20,000 that might attend a major comics or animation convention. Attendees may be drawn in from only the local area, or a larger region, or from all over the world. Most conventions are small enough that they are hosted in their entirety in a hotel. Some may occupy multiple Overflow Hotels in addition to the one hosting the main function space. Larger conventions may use a separate convention center or other function space. In that case, adequate hotel accomodations in the near vicinity of the convention site is essential. (Small local events, especially on college campuses, may get away without managing hotel accomodations at all, however.)
A compact site, or at least easy circulation on the site is important. If attendees have difficulty moving around at the convention site, or between their accomodations and the site, they will not take advantage of all the features and events. If they anticipate problems, some may not come at all.