OpenSim manifests itself in a few ways, the terminology can get confusing. Here is my attempt at clearing up confusion and misunderstandings when attempting to refer to the software, and it's various applications:
Grid - a community or network of individual simulators using some form of shared database for one or all of it's grid services. A collection of independent Opensim Instances, Simulators, Sims, Regions, or Megaregions sharing a database or databases which handle assets and inventory for grid members.
Instance - One example of Opensim running, on a server or PC, in what ever form as outlined below. Usually, but not always, contained inside a single directory.
Simulator, or sim for short - One instance of Opensimulator, running anywhere from one region to hundreds.
Standalone - A simulator or sim, running independently and not as part of a network of other simulators. Could be just a single region, or hundreds.
Standalone, Hypergrid mode - A standalone sim which is networked and accessable by teleporting from other standalones or from other simulators in a grid, by way of simple changes in the OpenSim.ini config file.
Region - Part of a Simulator, the main unit of a sim. Currently 256 x 256 meters, but other sizes are in the works for Opensim.
Parcel - A subunit of a Region.
Megaregion - a group of contiguous regions in one instance of Opensim without any restraints caused by borders. In effect, a group of anywhere from 2 to hundreds of regions configured to act as one single region. Note. Megaregions have been replaced by VAR regions in august 2016 and are no longer supported.
Var Region- a Var region is a Variable Size region. they can be 2x2 3x3 4x4 etc. They provide in a scalable environment without border crossings between the regions.
More info here
Ken Grunke aka Key Gruin