Originally, a large landmass usually referred to as the mainland. Current usage expands the definition to include groups of privately-owned sims (some suggest three or more, some five or more) that form a coherent land mass and maintain some common features--theme and ownership being most evident.
The scale of Second Life prevents a signficantly accurate application of the term "continent" to anything other than the entire Metaverse. With a sim side of 256 meters 100 sims laid end to end would represent about 25.6 kilometers, or about 17 miles.
In the U.S. a "standard" city block is about 10 acres, or about 2/3 the size of a Second Life sim.
The word Continent will be more appropriately applied to coming associative grids.