Here is the definition of WordPress from the top sites introducing this content management system or site builder.

1. The WordPress Foundation is a charitable organization founded by Matt Mullenweg to further the mission of the WordPress open source project: to democratize publishing through Open Source, GPL software.
2. WordPress is a popular content management system (CMS) that powers over 40% of all websites on the internet. It’s an open-source platform that allows users to create, manage, and modify their websites without needing extensive coding knowledge. Whether you’re a blogger, business owner, or developer, WordPress offers tools and features to suit your needs.
3. WordPress is a free, open-source website creation platform. On a more technical level, WordPress is a content management system (CMS) written in PHP that uses a MySQL database. In non-geek speak, WordPress is the easiest and most powerful blogging and website builder in existence today.

This is how WordPress is introduced on the official website

The open source publishing platform of choice for millions of websites worldwide—from creators and small businesses to enterprises.

In short and in a practical definition, WordPress is a free site builder software that allows you to design a site without coding.