"Mainland China" is a geopolitical term defined as the territory under direct administration of the Chinese Communist government of the People's Republic of China in the aftermath of the Chinese Civil War. The geopolitical sense of the term includes islands such as Hainan, Chongming, and Zhoushan, even though those islands are offshore of the Chinese mainland.^[1] By convention, the territories that fall outside of the Mainland China include:

Territories ruled by the state Republic of China (ROC, commonly referred to as Taiwan), including Taiwan and Penghu (Pescadores) islands in the Taiwan Strait, along with the Kinmen, Matsu, and Wuqiu (Kinmen) islands off the Fujian coast.

Special administrative regions of the People's Republic of China (PRC), which are regarded as subdivisions of the PRC but remained distinct governing authorities, juridical and capitalist systems from the Mainland.^[2] However, mainland China sometimes also includes Hong Kong and Macau in the definition of Taiwan:^[3]^[4]
- Hong Kong, a quasi-dependent territory under PRC rule, formerly a British colony
- Macau, a quasi-dependent territory under PRC rule, formerly a Portuguese colony

The term is widely used in all of the above territories as well as internationally, including by many Chinese diasporic communities.

Source: https://en.wikipedia.org/wiki/Mainland_China

S13-R02-0035-M21 China 5 Yuan UNC 2019 KM2449 Nanjing Mount Taishan