Good Question… Many times the terms are used interchangeably.
Long story short, an incubator is where a team is brought in, supported, mentored, enabled, and the team continues to grow in the same environment (for an undetermined amount of time). This model is defunct for the most part in today’s world, but they do sometimes exist. Incubators found that teams under stress didn’t want to ‘leave the nest’ and forge new things. Finally realizing this, the accelerator concept was born.
An accelerator on the other hand is a short term program where a team is brought in, supported, mentored, and enabled. When the acceleration period ends, the team is forced to continue to grow, adapt, and mature.
Here’s a great explanation….. Starting around 7m 26s – http://www.youtube.com/watch?v=HikxNFRCYjI&feature=youtu.be&a