IAP, or In-App Purchase, is a popular term used in the mobile app industry to refer to add-on purchases made by users within apps. Some synonyms for IAP include "in-app buying," "in-app transactions," "app upgrades," and "microtransactions." App developers often use IAP to monetize their apps and generate revenue from their user base. Other alternatives for IAP include "in-app sales," "in-app upgrades," or "in-app subscriptions." The growing popularity of mobile gaming has made IAP a significant revenue source for app developers. However, app developers must also ensure that their IAP systems work smoothly and are transparent to avoid user frustration and potential legal issues.