"Addin" is a commonly used term in software development and refers to a small program or add-on that extends the functionality of a larger software application. However, there are several other synonyms for "addin" that can be used interchangeably to explain the same concept. Some of these synonyms include "plugin", "extension", "module", "component" and "widget". While each of these terms may have slightly different connotations or specific uses, they generally refer to similar functionalities within a software application. Knowing these synonyms can help developers and users better understand the diverse range of additional features that can be added to a software program.