Circumstance Bonus

A Circumstance Bonus is a DM bonus (or penalty) imposed for favorable or unfavorable circumstance. It usually results in a +1 or -1 to the check. Circumstance bonuses do not stack, either things are favorable or unfavorable. However Circumstance Bonuses and Penalties do cancel each other out, similar to Advantage/Disadvantage.
This may cover many situations, and includes the use of special equipment. If a piece of equipment provides a circumstance bonus, it will say so in its write-up.
Examples: A Listening Horn provides a circumstance bonus when making Perception checks to listen at a door. The Tome known as the Maleficarum provides a circumstance bonus when researching information about devils and the Hells.