The Cambridge dictionary defines cross-border as something happening between different countries or involving people or businesses from different countries. When it comes to cross-border ecommerce, the term generally defines international online trade. It entails the sale or purchase of products via online shops across national borders. Buyer and seller are not located in the same country and are often not ruled by the same jurisdiction, use different currencies, and speak different languages.
Online trade within the EU, with its single market and common currency in many member states, is evenly referred to as cross-border ecommerce as for example selling from Germany to China.
Cross-border ecommerce can refer to online trade between a business (retailer or brand) and a consumer (B2C), between two businesses, often brands or wholesalers (B2B), or between two private persons (C2C), e.g. via marketplace platforms such as Amazon or eBay.