Definition of "mainlander"
A person from mainland China, as opposed to someone from Hong Kong, Macau or Taiwan.
Quotations
Since many factories have not been repaired or are operating at low capacity, there is much unemployment. Repatriation of overseas Taiwanese has aggravated the problem, which the government has made no real effort to solve. During the war the Japanese built at Chiayi a large factory for making high-octane gas. Now it is closed, and one-fifth of the town's population are out of work. It is not surprising that at Chiayi last month there was fierce fighting against the mainlanders.
1947 June 7, Peggy Durdin, “Taiwan : China's Unhappy Colony”, in The Nation, volume 164, number 23, New York, page 686