What is headless eCommerce?

It's a distribution of all goods a company sells across multiple stores and warehouses with no single application serving as the "central" authority from which other applications could pull the data.

It's a sales strategy based on the company not having any eCommerce website of their own, relying on other vendors selling its goods online and having a system with API that allows fulfilling orders on their behalf with their branding.

It is a separation of the front end and back end of an eCommerce application, with the back end exposing an API to which one or multiple front end applications can connect.