A bonus granted because of specific conditional factors favorable to the success of the task at hand. Circumstance bonuses stack with all other bonuses, including other circumstance bonuses, unless they arise from essentially the same benefit. For instance, a magnifying glass gives a +2 circumstance bonus on Appraise checks involving any item that is small or highly detailed, such as a gem. If you had a second tool that also granted a circumstance bonus from improved visual acuity (such as a jeweler's loupe), the circumstance bonuses wouldn't stack.